(I didn't know this so I thought I'd share!)
• How does BabyCenter count the weeks of pregnancy?Does BabyCenter count the weeks of pregnancy starting from conception or from a woman's last menstrual period?
At BabyCenter, we count the weeks of pregnancy starting from the first day of your last menstrual period (also called your "LMP") because that's the way most medical practitioners do it these days. A few may count pregnancy from your estimated date of conception, which is the day that you were likely to be ovulating during the month you became pregnant. (This is usually about two weeks after your last period started.) While the estimated day of conception is literally the day you first became pregnant, most practitioners don't count pregnancy from that date because there's no way to know for sure when you really did ovulate and conceive. So they take the easier route and go back to the first day of your last menstrual period. Then they count 40 weeks from that day to find your due date. (On the other hand, if you're counting pregnancy from your estimated date of conception, your due date will be only 38 weeks from that day.) And by the way, your last menstrual period is also the date from which your practitioner will measure your baby's gestational age. That is, when you're 20 weeks pregnant (starting from your LMP), your baby's gestational age is also 20 weeks.