Calculating Your Due Date

There are generally two ways of tracking a pregnancy -- from your last menstrual period (LMP) or by using 28-day "lunar months" starting from an estimated date of conception. The most commonly used method of tracking a pregnancy and calculating a due date uses the last menstrual period as a starting point.

 

When using your last menstrual period to calculate your due date, the pregnancy lasts an average of 280 days, or 40 weeks. To calculate your pregnancy due date using your last menstrual period, your healthcare provider will add nine months to the first day of your last menstrual period and then add seven days to that date, or he or she will count ahead 280 days from the beginning of your last menstrual period. For example, if your LMP was June 12, add nine months (March 12) then add seven days, arriving at a due date of March 19. Healthcare practitioners usually have a "pregnancy wheel," which does the calculation for them.

 

Another way to calculate your due date involves the use of 28-day lunar months. Instead of starting with your LMP, when using lunar months to calculate your due date, you start with an estimated date of conception -- about two weeks later than the first day of your last menstrual period. Therefore, when using lunar months, you're tracking the pregnancy over 266 days, or 38 weeks. This also means that when a woman is 33 weeks pregnant when using lunar months to track her pregnancy, she is 35 weeks pregnant based on the more popular LMP method.

 

As a historical note, lunar months were 28 days -- measured by the revolution of moon around the earth (hence "lunar"). This method considers each month 28 days -- meaning your pregnancy actually lasts 9½ lunar months.

 

Whether you use last menstrual period method or lunar months to track your pregnancy, remember that your due date is approximate, and you should expect your baby anywhere from two weeks before to two weeks after that due date. Only about 5 percent of babies are actually born on their expected due date.