I have agonized over this at great length, at:
among other places. I do not know. But it seems to me that, as long as we are not talking about Abortion, we act as though we think the fetus gradually becomes a human being over the course of pregnancy. As best as science can tell, a good half or more of successful human conceptions to not implant. We don't care, we just flush them down the toilet as a heavy period. When a miscarriage occurs we are sad, but we do not view it as a tragedy like the death of a little child. And certainly, like you say, a newborn baby is a person so how could the baby have been otherwise at the start of labor?