like LauraB78 said pregnancy is 40 weeks - roughly a little less than 10 months - if you think about it the first 2 weeks your not actually pregnant (b/c they date pregnancy from first day last menstrual period and we don't conceive until 2 weeks later than that)- which makes pregnancy 38 weeks. Then if you calculate into the figures that every calendar month is not exactly 28 days (all are 30 or 31 except february) it adds up roughly the other 2 weeks. Which is where I am assuming the people since the dawn of time have figured 9 months. A calendar month is nevery exactly 4 weeks - except for February.
B/c women are actually pregnant 10 months b,c of the way they have calculated things "since the dawn of time" as my Dr. says. It is confusing, though.. I am 29 weeks and due in January. When is your due date and what are you having?