This comes up often in communities where there aren't many parents. As a parent, I can tell you that every single doctor in the country uses the "40 weeks" sta

This comes up often in communities where there aren't many parents. As a parent, I can tell you that every single doctor in the country uses the "40 weeks" standard for preganancy, which counts from LMP (last menstrual period). Anybody who tells you "I'm 16 weeks pregnant" is counting from their last period. That means, under the "40 week" system, the first two weeks of "pregnancy" occur before conception. (Or, as pregnant people say, the first two weeks are free.) Yes, this is a silly system. But it dates back to a time in history when it was easy to know the date of your last period but hard to know your date of conception. Ultrasounds have changed that, but the "standard" 40-week calendar remains. Eg (in honor of About.com's transaction): http://pregnancy.about.com/od/pregnancycalendar/p/week3.htm http://pregnancy.about.com/od/pregnancycalendar/p/week3.htm