Once expectant parents have a 'positive' pregnancy test, their first question often is "When is my baby due?"  Pregnancy due dates are said to be 40 weeks long. The '40 week' estimation is based on the assumption that a woman's pregnancy lasts 10 lunar months (or 280 days), being about 9 calendar months.  However, as the first day of the woman's last menstrual period is used as a definite 'starting point' and conception typically happens 2 weeks after this time (or two weeks before the woman's next period is due) a pregnancy is really only 38 weeks long (or 266 days), being about 8 ½ calendar months. Therefore, one week after conception, a woman is said to be '3 weeks pregnant'.  Use our Due Date Calculator to help you calculate your due date in just seconds.