Duration of pregnancy

Though pregnancy begins at conception, it is more convenient to date from the first day of a woman's last menstrual period (LMP), or from the date of conception (if known). Starting from one of these dates, the expected date of delivery (EDD) can be calculated. Counting from the LMP, pregnancy usually lasts between 37 and 42 weeks, with the EDD at 40 weeks, 38 weeks after conception. 40 weeks is a little more than nine months and six days, which forms the basis of Naegele's rule for estimating date of delivery. Gestation peroid in humans is around 42 weeks (between 259 and 294 days since LMP).