The following three items concern when the product of human conception might be considered a "person." The concept of "personhood" is, of course, a social concept, not a scientific one. But the religious claims that a zygote (fertilized egg) should receive the rights of a person brings the public into an awareness of what biological science has to say about the time when a conception is an individual "person." The first reference is to a chapter from the book Bioethics and the New Embryology: Springboards for Debate (2005; Sinauer Associates) by Scott F. Gilbert, Anna Tyler, and Emily Zackin. The second reference is to an article published in the journal Embryo, and was taken from a lecture given at the Regina Apostolorum in Rome. The third reference is to a lecture by Scott Gilbert given to the American Reproductive Health Professional Society, 2010
While it is advised to see the evidence and the references in the original sources, a summary is given below.
What does contemporary science say about when an embryo becomes a person? There is no consensus among biologists as to this question. Below are four points in development at which different groups of scientists maintain that personhood may begin. Other scientists, however, say that the question itself is unscientific and cannot be answered.
Fertilization/Gastrulation Some scientists take a genetic approach, and suggest that life begins at fertilization, when one attains one's genome. This idea is also found among antiabortion activists who believe that at this point, as one website put it, "intelligence and personality— the way you look and feel—were already in place in your genetic code. At the moment of conception, you were essentially and uniquely you."
Historian Susan Lindee and sociologists Dorothy Nelkin have shown that we are constantly being told that DNA is our soul or essence—for example, "The sauna is in the DNA of every Finn."
But biologists with embryological leanings take issue with this idea. Identical twins, for instance, have the same DNA, but they certainly have different personalities and, theologically speaking, distinct souls. Twins can form up to the point of individuation, when animal embryos undergo a process called gastrulation, in which the cells are “told” what to become. In human embryos, this occurs around 14 days after fertilization. Thus, countries such as Britain and Canada allow research on embryonic stem cells, which pre-date individuation. The embryos used for stem cell research do not have a front, a back, a head, or limbs.
EEG activity Another group of biologists argues that personhood does not begin until brain activity can be recorded as an electroencephalogram, or EEG, around week 26. After all, the loss of that activity represents a loss of personhood to the extent that organs may be removed for transplant. This parallels the earlier Catholic view expressed by St. Thomas Aquinas, who said that ensoulment occurs sometime after conception, when one has developed the anatomy needed to house the human-specific, rational soul.
Viability/birth Still other biologists contend that only birth itself makes us physically distinct individuals, independent of maternal physiology. The anatomy of our heart, lungs, and blood vessels changes at our first breath.
This, interestingly, is where the Bible claims personhood originates. Genesis 9:6 says that one who murders a man must himself be destroyed. But Exodus 21:22 says a man who causes a woman to miscarry is not to be put to death, but rather should pay a fine. In the Bible, personhood is a birthright.
Anti-abortion advocates maintain that abortion (or the death of early embryos for stem cell research) is an affront to human dignity. It makes the human into a usable entity, a "thing," not a "who." This is an important concern. However, it works both ways. By equating a fertilized egg with an adult human, one not only makes the zygote like the person; it makes the person like the zygote. As less than half of normal human conceptions make it to term, most zygotes don't become babies. Zygotes can be cheap, and human life never should be.