Why do you add 2 weeks for gestation?

Answer 1

Its not like they just add 2 weeks, they measure from your last period and its 40 weeks from that. Most women have a 28 day (4 week) cycle and since you usually ovulate around the middle of your cycle and that's usually about the time you conceive, its like the basically add 2 weeks from when you probably conceived. So for purposes of counting, its sort of like your 2 weeeks pregnant when you conceive. seems crazy but that's how they do it.