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Monozygotic Twinning and Abortion

Does monozygotic twinning prove that life begins after conception?

Monozygotic Twinning and Abortion: Does monozygotic twinning prove that life begins after 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:

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?

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  1. Ronan O'Rahilly and Fabiola Muller, Human Embryology & Teratology (New York: Wiley-Liss, 2001), 84.

Continue to the next page: Prenatal Development

Parent Page:

  • Medical Testimony: A new human being comes into existence during the process of fertilization.

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