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Monozygotic Twinning and Abortion
Does the existence of monozygotic twinning prove that life doesn't begin at conception?
Page Summary:
While the zygote's ability to reproduce itself early in pregnancy raises some ethical questions, there are plenty of reasons why this doesn't change the fact that individual human development begins at conception.
About one-third of all twins are monozygotic, or "identical." This means they begin as a single zygote but divide into two zygotes somewhere between 3-6 days after conception. In extremely rare instances, this division can occur as late as 12 days from conception.1 Biologists don't know exactly how or why this happens, and unlike fraternal (dizygotic) twinning, monozygotic twinning does not seem to be influenced by heredity or environment. How does this phenomena relate to abortion? Some argue that because one human zygote has the capacity to become two (or more) human zygotes, that is evidence that individual human life does not begin at conception.
Before considering such a claim, it should be pointed out that surgical and medical abortions are both performed well after the zygotic stage of pregnancy has ended. So if you're going to argue that human zygotes should not be recognized as persons until after the capacity for twinning has ceased, be aware that this argument does nothing to justify abortion in the mainstream. Where it does come into play is in the joint arenas of birth control, embryonic stem cell research, and in vitro fertilization. All three interact with the human embryo during the zygotic stage. Specifically, certain birth control methods can destroy an already conceived human embryo by preventing implantation. Embryonic stem cell research relies on extracting embryonic stem cells that can only be obtained by destroying a human embryo, and it is commonplace for "extra" human embryos to be discarded during in vitro fertilization.
The question then is this. Does the existence of monozygotic twinning prove that the human embryos being destroyed during the zygotic stage are not actually persons? To help answer that question, there are a number of things to consider:
- There is overwhelming, biological consensus that individual, human life begins at conception. It is the only definitive starting point in human development – with the possible exception of the splitting that occurs during monozygotic twinning itself.
- Parthenogenesis is not a process by which one organism becomes two. It is a process by which one organism produces another. If this is indeed the process by which monozygotic twinning takes place, there is a very real sense in which one twin is "parenting" the other.
- If you understand monozygotic twinning to be a form of parthenogenesis, it is reasonable to conclude that the life of one twin began at conception while the other's began at the point of division.
- In the broader realm of cloning, the production of a second, genetically-identical clone does not mean the original being ceases to exist or never existed at all.
- The process of twinning does not appear to alter the essential character of the zygote that existed before the twinning occurred. Where there had only been one "body," cleavage yields two.
- The process of human reproduction is itself a remarkable example of how one body can give rise to a separate, morally-significant, genetically-distinct, second body – without ever ceasing to exist itself. Conceiving and giving birth doesn't mean the woman wasn't already human before the process took place. Nor does anyone suggest that she has somehow become two people. Her body has simply produced another body.
- Science can tell us when individual, biological life begins. It cannot tell us when "personhood" begins or "ensoulment" takes place.
- Since there is no way to distinguish between zygotes that will divide into monozygotic twins and those that won't, and since the division can occur anywhere within a nine-day range, the existence of monozygotic twinning does not provide an observable line of demarcation whereby an embryo definitively moves from being a human non-person to being a human person.
- The attempt to make a distinction between human beings and human persons has never turned out well. The most ethically-safe criteria is to regard all living human-beings as persons under the law.
Philosophically, the reality of monozygotic twinning does raise some perplexing questions. But from a biological standpoint, not much changes. Even if you want to use monozygotic twinning to argue against the existence of "personal" human life prior to day twelve, does it really make sense to argue that so long as one human being can become two human beings, we should be allowed to destroy the one before that can happen? Shouldn't the human embryo's remarkable capacity to reproduce itself secure it more protection instead of less?
- Ronan O'Rahilly and Fabiola Muller, Human Embryology & Teratology (New York: Wiley-Liss, 2001), 84.
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