I'll tell you the main thing for me about when life begins.
I've had 3 miscarriages, and 2 successful pregnancies.
After the 2nd miscarriage, I stopped thinking about it being a baby until I saw a heart beating. Before that, there is a 33% chance that a pregnancy will miscarry. After the heart starts beating, there is only a 1 % chance that a pregnancy will miscarry. So when the heart starts beating is when I think it changes from being a lump of cells, to actually being a baby.
As a mother of 5 on earth and one in heaven, who died at birth, I find it emotionally impossible, but mentally possible to understand a woman killing her child. I think it is with some ignorance, others pragmatism; with some selfishness, and with others a religion that delights in defying God.