The Length of Pregnancy
The average length of pregnancy is 280 days. This is about 9 calendar months or 10 lunar months. A lunar month is the time it takes to go from full moon to full moon—28 days.
Your doctor calculated your estimated due date by adding 7 days to the first day of your last normal menstrual period and then counting back 3 months. For example, if the first day of your last menstrual period was February 15, he added 7 days, which brought the date to February 22, then subtracted 3 months, for an expected due date of November 22. This is about 280 days after February 15. The baby would be only about 266 days old on November 22, but the date of the last menstrual period is used because the actual date of conception is usually unknown and could be as early as day 5 or 6 of the menstrual cycle. Only about 4 percent of women carry their babies for exactly 280 days, but 66 percent deliver within 10 days before or after their due dates. The due date is only an estimate, based on an average of all pregnancies. Your baby may take more or less time to develop, the same way that some babies take more or less time to get their first tooth.