Biologist Scott Gilbert, an expert in human development, tells us that there are at least four distinct moments that can be thought of as the beginning of human life. Each can be said to be biologically accurate. The Genetic view
(the position held by the Roman Catholic Church and many religious conservatives) holds that life begins with the acquisition of a novel genome; it is a kind of genetic determinism.
The Survival Doctor's Guides to Burns and Wounds, by @James HubbardThose who hold the Embryologic view
think life begins when the embryo undergoes gastrulation, and twinning is no longer possible; this occurs about 14 days into development. (Some mainline Protestant religions espouse a similar view.)
Proponents of the Neurological view
adhere to brainwave criteria; life begins when a distinct EEG pattern can be detected, about 24 to 27 weeks. (Some Protestant churches affirm this.) Interestingly, life is also thought to end when the EEG pattern is no longer present.
Finally, one can say that life begins at or near birth
, measured by fetal viability outside the mother’s body. (Judaism affirms something close to this position.) After all, somewhere between 50 and 60 percent of all embryos conceived miscarry.
So, when does life begin? I do not think we can know this with any more certainty than we know when life ends. People of faith, and people of good conscience, are going to have to agree to disagree—with a good dose of humility—on matters of life and death.