Whether out of ignorance or cowardice, no Democratic candidate has countered the Republican candidates' insistence that life begins at fertilization. As an embryologist and the author of embryological textbooks, I can say with absolute assurance: There is no consensus among embryologists as to when an individual human life begins.
Embryologists fall into five different groups when it comes to the start of human life.
One group of embryologists claims that human life begins at fertilization. These scientists see the genome as the essence of life, and they often talk about DNA as if it constituted some sort of soul.
Anti-abortion and zygote-rights websites are full of statements insisting that every attribute of our life -- intelligence, attractiveness, outgoingness -- is determined by the genes that we get when sperm meets egg. This is definitely not the case. Any mother of identical twins (whose genes are the same) knows this. The current science of epigenetics has put this knowledge on a molecular basis, showing that environmental agents play major roles in who we are.
Another group of embryologists champions a stage of development called gastrulation or individuation, because this is the stage when an embryo loses its ability to form twins and triplets. At gastrulation, about 12-14 days into human development, the fates of our cells are determined, and an embryo can become only one adult.
This is also about the same time that the embryo begins implanting into the uterus and thereby initiates pregnancy. The view that gastrulation is the beginning of an individual life has been popular in Great Britain, where embryologist Lewis Wolpert has forcefully commented, "It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life." Moreover, gastrulation makes it clear that if one is debating when a person receives a soul (something scientists don't really argue about), fertilization is a bad starting place. This is because the fertilized egg can become several embryos.
Do they have the same soul? Does the soul split? Catholic theologians (who do argue about such matters) have had a rough time with this question.
A third group of embryologists claims that human life begins around week 24-28, when the human-specific electroencephalogram (EEG) waves are seen. This marks the physiological states necessary for consciousness. In America, we say that the loss of the EEG (i.e., flat-lining) is the end of a human life, even though other organs are still functioning. Thus, according to this view, if we are willing to accept the loss of the EEG pattern as the death of a person, such personhood would arise when the EEG pattern was established. This period of EEG acquisition is also the period where the fetus becomes viable were it to be born prematurely.
A fourth group of embryologists claims that human life begins at or near the time of birth. This physiological claim notes that while the embryo is within the mother, it is unable to function on its own. The human heart does not form fully until after the first breath, and the passage from uterus to the outside world is fraught with danger. These embryologists also point out a fascinating statistic: Most fertilized eggs do not live to be born. Between 60-80 percent of all human conceptions die before birth. Many conservatives equate fertilization with birth and claim that once fertilization occurs, birth is expected. No, it is not.
Finally, there is the fifth position that finds the question about when individual human life begins to be unscientific, if not silly. One of America's foremost biologists, Theodosius Dobzhansky, put it this way, "The wish felt by many people to pinpoint such a stage probably stems from the belief that a soul, conceived as a preternatural entity, descends upon a formerly soulless living stuff, and suddenly transforms the latter into human estate. I hope that modern theologians can accept the idea that the transformation is not sudden, but gradual."
Simply put, there is no consensus among scientists as to when human life begins. Therefore, any politician who bases a position on abortion on what science "knows" is at best uninformed. I don't know where some of the Republican candidates received their training in embryology, but their views are not based on contemporary science. Just as these candidates have ignored the sciences of global warming and evolution, they have also ignored the science of embryology.