How accurate is your due date?

As soon as you find out you’re pregnant, the first thing that comes to mind is, “When will my baby be here?” Of course, calculating the exact date that your little one will arrive is a very inexact science. So when the doctor tells you the news, just how accurate can you expect it to be?

Your due date is calculated from the date of your last menstrual period, or LMP. Most women ovulate around the 14th day of their cycles, and the average pregnancy lasts 280 days, or 40 weeks, from the LMP (266 days from the actual date of conception). With that in mind, your due date is calculated by adding seven days to the last day of your last period, adding a year and subtracting three months. For example, if July 10, 2011 was the date of your LMP, your expected due date would be April 17, 2012.

Of course, not every woman’s menstrual cycle is exactly 28 days long, and not all women ovulate on the 14th day of their cycle. Additionally, a healthy pregnancy can last anywhere from 37 to 42 weeks, meaning predicting the exact arrival of your little bundle of joy is all but impossible. In fact, only 4 percent of babies are born on the date predicted by the doctor, although most are delivered within a week or two of the expected day.

Not knowing when your baby will arrive is part of the fun of pregnancy, so embrace this special and unique time in your life and let your body do what it was designed to do.