The law isn't always based on science, because the issues are social. Our constitution isn't concerned when life begins; it is concerned with the rights of born or naturalized citizens--something a fetus is not. Pregnant women ARE, and therefore their rights take precedent over all other concerns.
The only true and exact scientific measurement of life beginning is at conception. Otherwise any other point is just conjecture not based on real science.
If law wnats to go around science it will. It has throughout man's history.
That being said, all th epagan proabortion nonsense can't escape the inevitible truth that the only definitive point of origin of a new human life is in fact the moment of conception.
Yabber and blather all you want but you know that to be true.