Opinion
LUCAS, C. J.
Penal Code section 187, subdivision (a), provides that “Murder is the unlawful killing of a human being, or a fetus, with malice aforethought.” (All further statutory references are to the Penal Code unless otherwise indicated.) In this case, we consider and reject the argument that viability of a fetus is an element of fetal murder under the statute. As will appear, however, we also conclude that this holding should not apply to defendant herein. Accordingly, we will affirm the judgment of the Court of Appeal.
Facts
On March 1, 1991, Maria Flores, who was between 23 and 25 weeks pregnant, and her 20-month-old son, Hector, went to a check-cashing store to cash her welfare check. As Flores left the store, defendant pulled a gun from the waistband of his pants and demanded the money ($378) in her purse. When she refused to hand over the purse, defendant shot her in the chest. Flores dropped Hector as she fell to the floor and defendant fled the scene.
Flores underwent surgery to save her life. Although doctors sutured small holes in the uterine wall to prevent further bleeding, no further obstetrical surgery was undertaken because of the immaturity of the fetus. The next day, the fetus was stillborn as a direct result of its mother’s blood loss, low blood pressure and state of shock. Defendant was soon apprehended and charged with assaulting and robbing Flores, as well as murdering her fetus. The [801]*801prosecution charged a special circumstance of robbery-murder. (§ 190.2, subd. (a).)
At trial, the prosecution’s medical experts testified the fetus’s statistical chances of survival outside the womb were between 7 and 47 percent. The defense medical expert testified it was “possible for the fetus to have survived, but its chances were only 2 or 3 percent.” None of the medical experts testified that survival of the fetus was “probable.”
Although section 187, subdivision (a), does not expressly require a fetus be medically viable before the statute’s provisions can be applied to a criminal defendant, the trial court followed several Court of Appeal decisions and instructed the jury that it must find the fetus was viable before it could find defendant guilty of murder under the statute. The trial court did not, however, give the standard viability instruction, CALJIC No. 8.10, which states that: “A viable human fetus is one who has attained such form and development of organs as to be normally capable of living outside of the uterus.” The jury, however, was given an instruction that allowed it to convict defendant of murder if it found the fetus had a possibility of survival: “A fetus is viable when it has achieved the capability for independent existence; that is, when it is possible for it to survive the trauma of birth, although with artificial medical aid.” (Italics added.)
The jury convicted defendant of murder of a fetus during the course of a robbery (§ 187, subd. (a); § 190.2, subd. (a)(17)(i)), assault with a firearm (§245, subd. (a)(2)) and robbery (§ 211). The jury found that, in the commission of each offense, defendant personally used a firearm. (§ 12022.5, subd. (a).) The jury found true the special circumstance allegation. Accordingly, because the prosecutor did not seek the death penalty, defendant was sentenced to life without possibility of parole, plus five years for the firearm use.
On appeal, defendant contended that the trial court prejudicially erred by not instructing the jury pursuant to CALJIC No. 8.10. He relied on United States Supreme Court decisions that have defined viability of a fetus in terms of “probabilities, not possibilities,” when limiting a woman’s absolute right to an abortion. (See Roe v. Wade (1973) 410 U.S. 113,163 [35 L.Ed.2d 147, 182-183, 93 S.Ct. 705] [defining viability as that point in fetal development when a fetus, if born, would be capable of living normally outside the womb]; Planned Parenthood v. Casey (1992)_U.S._[120 L.Ed.2d 674, 112 S.Ct. 2791] [reaffirming Roe’s viability definition].) By analogy to the abortion cases, defendant asserted that a fetus is not viable under section 187, subdivision (a), unless “there is a reasonable likelihood of [its] sustained survival outside the womb, with or without artificial support.” (Colautti v. Franklin (1979) 439 U.S. 379, 388 [58 L.Ed.2d 596, 605, 99 S.Ct. [802]*802675].) Thus, defendant claimed, rather than defining viability as a “reasonable possibility of survival,” the trial court should have instructed the jury under the higher “probability” threshold described in CALJIC No. 8.10.
The People argued that no viability instruction was necessary because prosecution under section 187, subdivision (a), does not require that the fetus be viable. After reviewing the wording of section 187, subdivision (a), its legislative history, the treatment of the issue in other jurisdictions, and scholarly comment on the subject, the Court of Appeal agreed with the People that contrary to prior California decisions, fetal viability is not a required element of murder under the statute. Nonetheless, the court reversed defendant’s murder conviction and set aside the special circumstance finding, on the ground that application to defendant of its unprecedented interpretation of section 187, subdivision (a), would violate due process principles.
As explained below, we agree with the People and the Court of Appeal that viability is not an element of fetal murder under section 187, subdivision (a), and conclude therefore that the statute does not require an instruction on viability as a prerequisite to a murder conviction. In addition, because every prior decision that had addressed the viability issue had determined that viability of the fetus was prerequisite to a murder conviction under section 187, subdivision (a), we also agree with the Court of Appeal that application of our construction of the statute to defendant would violate due process and ex post facto principles. (People v. King (1993) 5 Cal.4th 59, 80 [19 Cal.Rptr.2d 233, 851 P.2d 27] [unforseeable enlargement of a criminal statute operates in manner of ex post facto law].) Accordingly, we address the instructional issue raised by defendant and agree with the Court of Appeal that the trial court prejudicially erred when it instructed the jury contrary to then-existing law, pursuant to a modified version of CALJIC No. 8.10. TÍius, we conclude we should affirm the Court of Appeal judgment in its entirety (affirming the assault and robbery counts and reversing the judgment of murder).
Discussion
I. Historical development
In 1970, section 187, subdivision (a), provided: “Murder is the unlawful killing of a human being, with malice aforethought.” In Keeler v. Superior Court (1970) 2 Cal.3d 619 [87 Cal.Rptr. 481, 470 P.2d 617, 40 A.L.R.3d 420], a majority of the court held that a man who had killed a fetus carried by his estranged wife could not be prosecuted for murder because the [803]*803Legislature (consistent with the common law view) probably intended the phrase “human being” to mean a person who had been born alive.
The Legislature reacted to the Keeler decision by amending the murder statute, section 187, subdivision (a), to include within its proscription the killing of a fetus. (Stats. 1970, ch. 1311, § 1, p. 2440.) The amended statute reads: “Murder is the unlawful killing of a human being, or a fetus, with malice aforethought.” (§ 187, subd. (a).) The amended statute specifically provides that it does not apply to abortions complying with the Therapeutic Abortion Act, performed by a doctor when the death of the mother was substantially certain in the absence of an abortion, or whenever the mother solicited, aided, and otherwise chose to abort the fetus. (§ 187, subd. (b).)
The legislative history of the amendment suggests the term “fetus” was deliberately left undefined after the Legislature debated whether to limit the scope of statutory application to a viable fetus. (Comment, Is the Intentional Killing of an Unborn Child Homicide? (1970) 2 Pacific L.J. 170, 174.) The Legislature was clearly aware that it could have limited the term “fetus” to “viable fetus,” for it specifically rejected a proposed amendment that required the fetus be at least 20 weeks in gestation before the statute would apply. (Assem. Bill No. 816 (1970 Reg. Sess.).)
In 1973, the United States Supreme Court issued a decision that balanced a mother’s constitutional privacy interest in her body against a state’s interest in protecting fetal life, and determined that in the context of a mother’s abortion decision, the state had no legitimate interest in protecting a fetus until it reached the point of viability, or when it reached the “capability of meaningful life outside the mother’s womb.” (Roe v. Wade, supra, 410 U.S. at p. 163 [35 L.Ed.2d at p. 183].) The court explained that “[viability is usually placed at about seven months (28 weeks) but may occur earlier, even at 24 weeks.” (Id. at p. 160 [35 L.Ed.2d at p. 181].) At the point of viability, the court determined, the state may restrict abortion. (Id. at p. 163 [35 L.Ed.2d at pp. 182-183].)
Thereafter, in People v. Smith (Karl Andrew) (1976) 59 Cal.App.3d 751 [129 Cal.Rptr. 498] (hereafter K.A. Smith), the Court of Appeal construed the term “fetus” in section 187, subdivision (a), to mean a “viable fetus” as defined by Roe v. Wade, supra, 410 U.S. at pages 162-164 [35 L.Ed.2d at pages. 182-183], In K.A. Smith, the defendant had beaten his wife, who was 12 to 15 weeks pregnant, saying he did not want the baby to live. The wife miscarried the ferns as a direct result of the beating. At trial, the parties stipulated that the fetus was not viable at the time of the miscarriage. (59 Cal.App.3d at pp. 753-754.)
[804]*804In affirming the trial court’s dismissal of the murder charge on the ground the fetus was not viable at the time of its death, the K.A. Smith court held that viability is an essential element of murder under section 187, subdivision (a), reasoning that “one cannot destroy independent human life prior to the time it has come into existence.” (K.A. Smith, supra, 59 Cal.App.3d at p. 756.)
Relying on Roe v. Wade, supra, 410 U.S. 113, the K.A. Smith court noted a mother’s constitutional right to abort the fetus during the first trimester of gestation and her qualified right to do so during the second trimester, and observed that until viability, the state has no interest in protecting the fetus from either abortion or murder. The court concluded that, “[i]mplicit in Wade is the conclusion that as a matter of constitutional law the destruction of a nonviable fetus is not a taking of human life. It follows that such destruction cannot constitute murder or other form of homicide, whether committed by a mother, a father . . . or a third person.” (K.A. Smith, supra, 59 Cal.App.3d at p. 757.) The K.A. Smith court defined viability as “ ‘having attained such form and development of organs as to be normally capable of living outside the uterus.’ ” (Id. at p. 758, quoting Webster’s New Internat. Dict. (3d ed. 1966) p. 2548.)
Subsequent cases also inferred a viability limitation to convicting a defendant of fetal murder under section 187, subdivision (a). In People v. Apodaca (1978) 76 Cal.App.3d 479 [142 Cal.Rptr. 830], involving an appeal from a conviction of fetal murder, the Court of Appeal rejected the defendant’s contention that section 187, subdivision (a), is unconstitutionally vague because it does not specify the requisite stage of development of the fetus covered by the section. (Apodaca, supra, at p. 485.) The court also rejected the defendant’s alternative argument that the state’s power to charge a defendant with the murder of an unborn child should be limited to a fetus that is viable when it is killed. (Id. at p. 487.) The Apodaca court held that it need not reach the constitutional question whether the trial court had erred in failing to define the word “fetus” in terms of viability, because uncontroverted medical testimony had indicated during trial that the fetus was viable at the time it was murdered. (Id. at p. 489.)
In People v. Smith (Robert Porter) (1987) 188 Cal.App.3d 1495 [234 Cal.Rptr. 142] (hereafter R.P. Smith), the defendant was accused of the double murder of a woman and her fetus. Over the defendant’s objection, the trial court had instructed the jury pursuant to a separate instruction submitted by the People that defined a fetus as “ ‘a viable unborn child.’ ” (Id. at p. 1513.) On appeal after the conviction, the defendant complained that the People’s instruction failed to define the term “ ‘viable’ ” and that “without a definition of viability the jury could have interpreted the instructions to [805]*805mean that the question of viability would be [answered by facts showing] that the [fetus] was alive at the time of the killing.” (Id. at p. 1513.) The defendant insisted that the trial court should have defined the term viable to mean an “ ‘unborn child . . . capable of independent existence outside the mother.’ ” (Ibid.)
The R.P. Smith court reviewed the earlier K.A. Smith and Apodaca decisions and concluded that the “term viable is at once simple to understand yet elusive. For this court to hold that the term viable has a common and ordinary meaning in everyday usage, we would have to take judicial notice of that fact. We must decline the invitation to do so. [][] The trial court should have instructed the jury as to the legal definition of. . , viable. The trial judge has a sua sponte duty to instruct the jury as to all essential elements of the charged offense. [Citation.] Viability of a fetus is a constitutional prerequisite for murder of a fetus by logical extension of [Roe] v. Wade, supra, 410 U.S. 113. The trial court erred in failing to instruct the jury as to the legal meaning of the term viable.” (R.P. Smith, supra, 188 Cal.App.3d at p. 1514.) Although the court held that the trial court erred in not instructing the jury under the “legal definition” of the term viable, it concluded the defendant had not suffered prejudice because “the record clearly demonstrate[d] the existence of the element of viability of the fetus.” (Ibid.)
A more recent Court of Appeal decision addressing fetal murder is People v. Henderson (1990) 225 Cal.App.3d 1129 [275 Cal.Rptr. 837] (hereafter Henderson). There, the defendant argued that his second degree murder conviction for the killing of a fetus should be reversed because section 187, subdivision (a), is unconstitutionally vague. The defendant asserted that the viability requirement imposed by decisional law “is so vague that it fails to provide notice to a perpetrator that his or her violent act may be prohibited by this statute.” (Henderson, supra, 225 Cal.App.3d at p. 1157.)
The Henderson court emphasized that the statute is not vague because it contains no ambiguities. In fact, the court observed, “the statute itself makes no reference whatsoever to viability. It is decisional law interpreting section 187 which limits the criminal liability for its violation to viable fetuses.” (Henderson, supra, 225 Cal.App.3d at p. 1158, italics added.) The court then determined that the definition of viability has been well established. Relying on R.P. Smith, supra, 188 Cal.App.3d 1495, the Henderson court concluded that a fetus is viable “ ‘when it has achieved the capability for independent existence.’ ” (Henderson, supra, 225 Cal.App.3d at p. 1157.)
This court has never directly addressed whether viability is a prerequisite to fetal murder under section 187, subdivision (a). We did, however, refuse [806]*806to consider whether an instruction on viability was sufficient in People v. Hamilton (1989) 48 Cal.3d 1142 [259 Cal.Rptr. 701, 774 P.2d 730], There, defendant was convicted of the murder of his wife and seven-month-old fetus. The trial court combined language from K.A. Smith, supra, 59 Cal.App.3d at page 757, and People v. Apodaca, supra, 76 Cal.App.3d at page 487, and instructed the jury that it “ ‘must find beyond a reasonable doubt that the fetus was viable, that is, capable of independent existence or as having attained such form and development of organs as to be normally capable of living outside the uterus. A fetus is deemed viable when it is possible for it to survive the trauma of birth, although with artificial medical aid.’ ” (Hamilton, supra, 48 Cal.3d at p. 1171, italics added.)
On appeal, the Hamilton defendant contended that the trial court’s instruction was contradictory and misled the jury into believing it could find him guilty of murdering a nonviable fetus. The defendant asserted that the jury should have been instructed pursuant to the United States Supreme Court’s pronouncement on a woman’s constitutional right to an abortion in Roe v. Wade, supra, 410 U.S. 113, and the subsequent definition of viability adopted by the Court of Appeal in Colautti v. Franklin, supra, 439 U.S. at page 388 [58 L.Ed.2d at pages 604-605], that a fetus is not viable under our murder statute unless “ ‘there is a reasonable likelihood of [its] sustained survival outside the womb, with or without artificial support.’ ” (Hamilton, supra, 48 Cal.3d at p. 1171.) We determined that we need not reach the merits of the claim, because there was uncontradicted evidence that the fetus had attained viability under any accepted test. (Id. at pp. 1171-1173.) Hence, we have never determined whether our Courts of Appeal have properly included viability as an element of the crime of fetal murder.
II. Statutory interpretation
Defendant asserts that section 187, subdivision (a), has no application to a fetus not meeting Roe v. Wade's definition of viability. Essentially, defendant claims that because the fetus could have been legally aborted under Roe v. Wade, supra, 410 U.S. at page 163 [35 L.Ed.2d at pp. 182-183], at the time it was killed, it did not attain the protection of section 187, subdivision (a). Defendant relies on K.A. Smith, supra, 59 Cal.App.3d 751, and its progeny to assert that if a fetus has not attained “independent human life” status under Roe v. Wade, supra, 410 U.S. at page 163 [35 L.Ed.2d at pages 182-183], it has not achieved “viability” under K.A. Smith, supra, 59 Cal.App.3d at page 759, and he therefore cannot be prosecuted under section 187, subdivision (a), for its murder.
But Roe v. Wade, supra, 410 U.S. 113, does not hold that the state has no legitimate interest in protecting the fetus until viability. Indeed, contrary to [807]*807the decisions in K.A. Smith, supra, 59 Cal.App.3d 751, People v. Apodaca, supra, 76 Cal.App.3d 479, R.P. Smith, supra, 188 Cal.App.3d 1495, and Henderson, supra, 225 Cal.App.3d 1129, Poe v. Wade principles are inapplicable to a statute (like section 187, subdivision (a)) that criminalizes the killing of a fetus without the mother’s consent. As observed by one commentator: “By holding that the Fourteenth Amendment does not cover the unborn, the Supreme Court was left with only one constitutionally mandated right, that of the mother’s privacy, to be considered along with the legitimate state interest in protecting an unborn’s potential life. The Roe decision, therefore, forbids the state’s protection of the unborn’s interests only when these interests conflict with the constitutional rights of the prospective parent. The Court did not rule that the unborn’s interests could not be recognized in situations where there was no conflict.” (Parness, Crimes Against the Unborn: Protecting and Respecting the Potentiality of Human Life (1985) 22 Harv. J. on Legis. 97, 144.)
Other scholarly comment agrees with Professor Parness. In her article, The Juridical Status of the Fetus: A Proposal for Legal Protection of the Unborn (1979) 77 Mich.L.Rev. 1647, 1678, Professor King states that, “Where the protectable interests of fully mature members do not conflict with those of less mature members, there is no justification for ignoring the latter’s claims. The Roe opinion was correct in recognizing a state’s legitimate interest in protecting the previable fetus. In . . . criminal law, when that interest does not oppose a protected interest of the mature mother, the state should not hesitate to vindicate it.”
Finally, as explained by Clarke Forsythe in Homicide of the Unborn Child: The Born Alive Rule and Other Legal Anachronisms (1987) 21 Val.U.L.Rev. 563, 616: “ ‘While the decision in Roe declares that the state may not protect the potential life of the human fetus from the moment of conception, it does so only in the very narrow context of the mother’s abortion decision.’ Under Roe v. Wade, therefore, the right to abortion is encompassed within the woman’s right to constitutional privacy. The fetus is not a ‘person’ for purposes of the Fourteenth Amendment and has no constitutional rights that would outweigh the exercise of the woman’s Fourteenth Amendment rights. The fetus’ rights and the state’s interest, or lack of interest, in protecting maternal health and in protecting the life of the fetus, were distinctly balanced against the woman’s right to privacy in the context of consensual abortion.” (Fn. omitted.) Thus, when the state’s interest in protecting the life of a developing fetus is not counterbalanced against a mother’s privacy right to an abortion, or other equivalent interest, the state’s interest should prevail.
Other states have adopted statutes that criminalize the killing of a fetus. Although no state has criminalized the nonconsensual killing of a “fetus,” [808]*808several states criminalize the nonconsensual killing of an “unborn child,” characterizing it as manslaughter or murder. In these states (Arizona, Illinois, Louisiana, Minnesota, North Dakota, and Utah), the murder statutes do not require that the unborn have reached a particular stage of development.1
The Illinois and Minnesota appellate courts have rejected equal protection and due process challenges to their feticide statutes. The challenges were based on the statutes’ asserted failure to distinguish between viable and nonviable fetuses. As discussed below, the arguments were rejected on the ground that protection of a woman’s privacy interest in the abortion context is not applicable to a nonconsensual murder of the unborn child.
In Illinois, the legislature eliminated an express viability requirement from its murder statute. The amended statute states that a “person commits the offense of intentional homicide of an unborn child” if he or she either intended to cause the death of or to do great bodily harm to the pregnant woman or her unborn child . . . [defined as “any individual of the human species from fertilization until birth].” (Ill. Rev. Stat. ch. 38, f 9-1.2(b)(l) (1987).) This statute was challenged in People v. Ford (1991) 221 Ill.App.3d 354 [163 Ill.Dec. 766, 581 N.E.2d 1189, 1197], involving a defendant convicted (under the amended statute) of killing his 17-year-old stepdaughter’s SVi-month-old fetus. (Id. at p. 1190.) The defendant contended the statute violated equal protection and due process principles because it failed to distinguish between a viable and nonviable fetus.
The Illinois Appellate Court held the statute does not violate equal protection principles because it does not affect any protectible interest held by the defendant, and it bears a rational relationship to a valid legislative purpose—protecting the “potentiality of human life.” (People v. Ford, supra, 581 N.E.2d at p. 1199, quoting Roe v. Wade, supra, 410 U.S. at p. 162 [35 L.Ed.2d at p. 182].) The court observed that a defendant who intentionally murders a fetus, and a pregnant woman who chooses to terminate her pregnancy, are not similarly situated. (People v. Ford, supra, at p. 1199.) “A woman has a privacy interest in terminating her pregnancy; however, defendant has no such interest. The statute simply protects the mother and the unborn child from the intentional wrongdoing of a third party.” (Id. at p. 1199.)
The Ford court also rejected the defendant’s due process/vagueness challenge that the “absence of statutory definitions of when ‘life’ begins and [809]*809‘death’ occurs will result in the trier of fact applying its subjective religious, philosophical, and political views to define those terms, thereby leading to arbitrary and discriminatory enforcement of the statute.” (People v. Ford, supra, 581 N.E.2d at p. 1200.) The court found the statute constitutional on the ground that it “only requires proof that, whatever the entity within the mother’s womb is called, it had life and, because of the acts of the defendant, it no longer does. The name given to that entity is irrelevant to the liability under the statute. The trier of fact will only be asked to determine whether the particular entity, whether an embryo, fetus, person, or human being, once had life and, because of the acts of the defendant, no longer does.” (Id. at p. 1201.)
The Illinois Appellate Court relied substantially on a decision of the Minnesota Supreme Court rejecting equal protection and due process challenges to a feticide statute by a defendant who had murdered a woman and her four-week-old embryo. (State v. Merrill (Minn. 1990) 450 N.W.2d 318.) The Minnesota Legislature includes in its definition of first degree murder the killing of “an unborn child with premeditation and with intent to effect the death of the unborn child or of another.” (Minn. Stat. § 609.2661(1) (1988).)
In rejecting the defendant’s constitutional challenge that the homicide statute violated equal protection because an unborn child lacks “personhood” and is not a “person" under Roe v. Wade, supra, 410 U.S. at page 158 [35 L.Ed.2d at page 180], deserving protection under the Fourteenth Amendment, the Minnesota Supreme Court observed, “The focus of [Roe] was on protecting the woman from governmental interference or compulsion when she was deciding whether to terminate or continue her pregnancy. . . . Significantly, the Roe v. Wade court also noted that the state ‘has still another important and legitimate interest in protecting the potentiality of human life.’ ... In our case, the fetal homicide statutes seek to protect the ‘potentiality of human life,’ and they do so without impinging directly or indirectly on a pregnant woman’s privacy rights. [][] The state’s interest in protecting the ‘potentiality of human life’ includes protection of the unborn child, whether an embryo or a nonviable or viable fetus, and it protects, too, the woman’s interest in her unborn child and her right to decide whether it shall be carried in útero. The interest of a criminal assailant in terminating a woman’s pregnancy does not outweigh the woman’s right to continue the pregnancy. In this context, the viability of the fetus is ‘simply immaterial’ to an equal protection challenge to the feticide statute. [Citation.]” (State v. Merrill, supra, 450 N.W.2d at p. 322.)
Although the Illinois and Minnesota statutes specifically state that viability is not an element to be considered for their application, both Ford, supra, 581 N.E.2d 1189, and Merrill, supra, 450 N.W.2d 318, illustrate that criminalization of the killing of a fetus without regard to viability is not violative [810]*810of either privacy principles, equal protection, or due process considerations. Both cases also illustrate that the legislature is free to impose upon the killer of a fetus the same penalty as is prescribed for the murder of a human being (although in neither state has the legislature permitted application of the death penalty for the murder of a fetus).
Like Illinois and Minnesota, California is a “code” state, i.e., the Legislature has the exclusive province to define by statute what acts constitute a crime (§ 6), and statutory provisions must “be construed according to the fair import of their terms, with a view to effect [their] objects and to promote justice” (§ 4). Under these principles, like Illinois and Minnesota, we find no impediment to our Legislature protecting the “potentiality of human life” from homicide.
Finally, both Ford and Merrill expressly distinguish fetal homicide from the abortion issue. Our Legislature does the same. Abortion is specifically exempted from section 187 under subdivision (b)(3), which states that section 187 shall not apply if “the act was solicited, aided, abetted, or consented to by the mother of the fetus.”
We conclude, therefore, that when the mother’s privacy interests are not at stake, the Legislature may determine whether, and at what point, it should protect life inside a mother’s womb from homicide. Here, the Legislature determined that the offense of murder includes the murder of a fetus with malice aforethought. (§ 187, subd. (a).) Legislative history suggests “fetus” was left undefined in the face of divided legislative views about its meaning. (See Comment, Is the Intentional Killing of an Unborn Child Homicide?, supra, 2 Pacific L.J. 170,172-175.) Generally, however, a fetus is defined as “the unborn offspring in the postembryonic period, after major structures have been outlined.” (Sloane-Dorland Ann. Medical-Legal Dict. (1987) p. 281.) This period occurs in humans “seven or eight weeks after fertilization” (ibid.), and is a determination to be made by the trier of fact. Thus, we agree with the above cited authority that the Legislature could criminalize murder of the postembryonic product without the imposition of a viability requirement. We need not address whether different concerns might apply to an embryo. Accordingly, to the extent K.A. Smith, supra, 59 Cal.App.3d 751, and its progeny require a fetus to be viable in order for its murder to be prosecuted under section 187, subdivision (a), they rue the statute and should be disapproved.2
[811]*811III. Due process challenge
Although the Court of Appeal herein found, consistent with the foregoing analysis, that viability is not an element of fetal homicide under section 187, subdivision (a), it determined that its “redefinition” of the crime amounted to a “major change in the law” that, if applied to defendant, would violate due process principles. The Attorney General, while not conceding the point, does not address the Court of Appeal’s analysis and judgment on this issue. Defendant reasserts here that if we conclude viability is not an element of fetal murder, we are bound by due process principles not to apply our decision to him. We conclude that our interpretation of section 187, subdivision (a), should apply prospectively only and not to defendant.
A statute “ ‘which makes more burdensome the punishment for a crime, after its commission,’ ” violates article I, section 9, clause 3, of the United States Constitution as an ex post facto determination of criminal liability (Collins v. Youngblood (1990) 497 U.S. 37, 42 [111 L.Ed.2d 30, 38-39, 110 S.Ct. 2715], quoting Beazell v. Ohio (1925) 269 U.S. 167, 169-170 [70 L.Ed. 216, 217-218, 46 S.Ct. 68]), as well as its California counterpart, article I, section 9 of the state Constitution (Tapia v. Superior Court (1991) 53 Cal.3d 282 [279 Cal.Rptr. 592, 807 P.2d 434]). Correspondingly, an unforseeable judicial enlargement of a criminal statute, applied retroactively, operates in the same manner as an ex post facto law. (Bouie v. City of Columbia (1964) 378 U.S. 347, 354 [12 L.Ed.2d 894, 900, 84 S.Ct. 1697]; see also People v. Escobar (1992) 3 Cal.4th 740, 752 [12 Cal.Rptr.2d 586, 837 P.2d 1100]; People v. Wharton (1991) 53 Cal.3d 522, 586 [280 Cal.Rptr. 631, 809 P.2d 290].)
Thus, holding a defendant criminally responsible for conduct that he could not reasonably anticipate would be proscribed violates due process because the law must give sufficient warning so that individuals “may conduct themselves so as to avoid that which is forbidden.” (Rose v. Locke (1975) 423 U.S. 48, 50 [46 L.Ed.2d 185, 188, 96 S.Ct. 243].) Defendant’s ex post facto contention does not rest on a change in a statute, but on the fact that until his crime was prosecuted, the Courts of Appeal had required a showing of fetal viability before allowing a conviction under section 187, subdivision (a), to stand. (K.A. Smith, supra, 59 Cal.App.3d 751; People v. Apodaca, supra, 76 Cal.App.3d 479; R.P. Smith, supra, 188 Cal.App.3d 1459; Henderson, supra, 225 Cal.App.3d 1129.)
In support of his contention, defendant relies on our recent decision in People v. King, supra, 5 Cal.4th 59 (hereafter King), in which we held that our decision overruling In re Culbreth (1976) 17 Cal.3d 330, 333 [130 [812]*812Cal.Rptr. 719, 551 P.2d 23], could not be applied retroactively to the King defendant because to do so would “make the punishments for [defendant’s] crimes more burdensome after he committed them.” (King, supra, 5 Cal.4th at p. 80.)
In In re Culbreth, supra, 17 Cal.3d 330, we applied section 12022.5, subdivision (a), enhancements to a defendant who had shot and killed his common law wife, mother-in-law, and brother-in-law with a rifle. Section 12022.5, subdivision (a), provides in relevant part that “any person who personally uses a firearm in the commission or attempted commission of a felony shall, upon conviction of that felony or attempted felony, in addition and consecutive to the punishment prescribed for the felony or attempted felony of which he or she has been convicted, be punished” as a sentence enhancement. (See, In re Culbreth, supra, at p. 332.) The question was whether the sentence could be enhanced for both of the murders or whether only one enhancement could be imposed for all three murders. We held, “The legislative purpose of section 12022.5 has been described as deterrence, i.e., to deter the use of firearms on subsequent occasions. . . . But if all the charged offenses are incident to one objective and effectively comprise an indivisible transaction, then section 12022.5 may be invoked only once and not in accordance with the number of victims.” (In re Culbreth, supra, at pp. 333-334.)
In King, we overruled In re Culbreth's “single-occasion” rule after concluding it had no support in the statutory language. But we also noted that Culbreth had been applied consistently since its pronouncement by this court, so that retroactive application would make the punishment for defendant’s crimes more burdensome after he committed them. (King, supra, 5 Cal.4th at p. 80; In re Baert (1988) 205 Cal.App.3d 514, 518 [252 Cal.Rptr. 418] [refusing to apply retroactively our holding in People v. Anderson (1987) 43 Cal.3d 1104 [240 Cal.Rptr. 585, 742 P.2d 1306], which eliminated intent to kill as an element of the felony-murder special circumstance].) The same principles apply here.
As discussed above, several Courts of Appeal have erroneously implied a viability requirement into section 187, subdivision (a). Although we are not, as in King, supra, 5 Cal.4th 59, faced with reconsidering our own precedent, the fact that a viability requirement has consistently been read into section 187, subdivision (a), supports defendant’s assertion that our proposed holding creates an unforseeable judicial enlargement of a criminal statute. (Bouie v. City of Columbia, supra, 378 U.S. at p. 353 [12 L.Ed.2d at pp. 899-900]; Rose v. Locke, supra, 23 U.S. 48.) Accordingly, we now consider whether the trial court prejudicially erred by instructing the jury pursuant to a modified instruction on viability.
[813]*813IV. Instructional error
Defendant requested the jury be instructed pursuant to CALJIC No. 8.10, which defines fetal murder as follows: “Every person who unlawfully kills a fetus with malice aforethought is guilty of the crime of murder in violation of Section 187 of the Penal Code. ... [1] In order to prove such crime, each of the following elements must be proved: H] 1. A viable human fetus was killed. 2. The killing was unlawful, and 3. The killing was done with malice aforethought. . . . [f] A viable human fetus is one who has attained such form and development of organs as to be normally capable of living outside the uterus.” The comment to the instruction states: “The term ‘fetus’ as used in Penal Code, § 187, means a viable unborn child. People v. [K.A.] Smith[, supra,] 59 Cal.App.3d 751, 758-759.”
As noted above, the trial court gave instead the following modified version of CALJIC No. 8.10: “Within the meaning of Penal Code section 187, subdivision (a), as charged in Count One, a fetus is viable when it has achieved the capability for independent existence; that is, when it is possible for it to survive the trauma of birth, although with artificial medical aid.” (Italics added.)
The Court of Appeal below assumed that the trial court’s modification of CALJIC No. 8.10 was taken in part from the decision in People v. Apodaca, supra, 76 Cal.App.3d 479, where that court attempted to define viability in resolving the defendant’s contention that he did not murder a viable fetus. As previously discussed, the Apodaca court noted that “[a] fetus is viable when it has achieved the capability for independent existence; as we have indicated, a fetus is deemed viable when it is possible for it to survive the trauma of birth, although with artificial medical aid.” (Id. at p. 489.) Although Apodoca discussed viability of a fetus in a context other than instructional error, the Court of Appeal below concluded that its discussion nonetheless served as a model for the present trial court’s instructional modification.
In People v. Hamilton, supra, 48 Cal.3d 1142, we reviewed an instruction substantially identical to the modified version of CALJIC No. 8.10 given here, but, as noted above, declined to decide whether the instruction as given “contained a latent but prejudicial ambiguity” because it “improperly implied that a fetus is viable if it is capable of being bom alive, even if it could not have survived for a sustained period outside the mother’s womb.” (Hamilton, supra, 48 Cal.3d at p. 1171.) We determined that we “need not address the merits of defendant’s claim, since the ambiguity he posits cannot have been prejudicial.... Uncontradicted and conclusive evidence established that the likelihood of this fetus’s sustained survival was high.” (Id. at [814]*814p. 1172.) Thus, the fetus would have been considered viable under any instruction.
As the Court of Appeal below observed, the wording of CALJIC No. 8.10, defining viability as “normally capable of living outside of the uterus,” while not a model of clarity, suggests a better than even chance—a probability— that a fetus will survive if born at that particular point in time. By contrast, the instruction given below suggests a “possibility” of survival, and essentially amounts to a finding that a fetus incapable of survival outside the womb for any discernible time would nonetheless be considered “viable” within the meaning of section 187, subdivision (a). Because the instruction given by the trial court substantially lowered the viability threshold as commonly understood and accepted (as defined by Roe v. Wade, supra, 410 U.S. at pp. 162-164 [35 L.Ed.2d at pp. 182-183], K.A. Smith, supra, 59 Cal.App.3d at pp. 752-753, and its progeny), we conclude that the trial court erred in instructing the jury pursuant to a modified version of CALJIC No. 8.10.
The question then is whether it is reasonably probable a result more favorable to defendant would have been reached absent the instructional error. (People v. Watson (1956) 46 Cal.2d 818 [299 P.2d 243].) The record shows the weight of the medical testimony was against the probability of the fetus being viable at the point it was tilled. Defendant’s medical expert opined that it was “possible” for the fetus to have survived the trauma of an early birth, but that its chances for survival were about 2 or 3 percent. Unlike People v. Hamilton, supra, 48 Cal.3d at pages 1171, in which we rejected the defendant’s claim of instructional error after determining the fetus had a “better than 50 percent chance of survival” (or a probable chance of living outside the womb), in this case none of the medical experts who testified at defendant’s trial believed that the fetus had a “probable” chance of survival. Accordingly, because the evidence on the issue of viability erroneously supported the concept of the “possibility” of survival, and the jury was then instructed that viability means “possible survival,” the jury was misinformed that it could find the fetus was viable before it “ ‘attained such form and development of organs as to be normally capable of living outside the uterus.’ ” (K.A. Smith, supra, 59 Cal.App.3d at p. 758; CALJIC No. 8.10.) Had the jury been given CALJIC No. 8.10, it is reasonably probable it would have found the fetus not viable. We conclude, therefore, that defendant was prejudiced by the instructional error and the conviction of fetal murder must be reversed.
Conclusion
We conclude that viability is not an element of fetal homicide under section 187, subdivision (a). The third party killing of a fetus with malice [815]*815aforethought is murder under section 187, subdivision (a), as long as the state can show that the fetus has progressed beyond the embryonic stage of seven to eight weeks.
We also conclude that our holding should not apply to defendant and that the trial court committed prejudicial error by instructing the jury pursuant to a modified version of CALJIC No. 8.10. We therefore affirm the judgment of the Court of Appeal.3
Arabian, J., concurred.