Estimated Due Date: a calculation of the average gestational length

Once you know you are pregnant, one of the first things everyone will be asking is, "When is your due date?" And later on during your pregnancy, everything is likely to revolve around this all important date. But how is this due date actually calculated, and what is its true significance?

An estimated due date (or expected due date, or expected date of confinement), sometimes abbreviated as EDD, is normally calculated using the first day of your last menstrual period even if you know exactly when you conceived. Based on this, the LMP method of calculating your due date, your pregnancy will be assumed to last for 40 weeks... or exactly 280 days long.

Generally speaking, medical professionals will agree that a full-term pregnancy is between 38 and 42 weeks of pregnancy. Most people would naturally deliver within two weeks of their estimated due date, going either way. Somewhere around five percent of pregnant women actually give birth on their expected due date, and in many cases that will be "due" to a c-section or induction of labor.

So, when is too early to go into labor, and when is too late? Too early is a matter of discussion. Many babies who are born at 36 weeks will be fine without medical assistance, but some studies indicate that even 38 weekers have more problems than 40 weekers, statistically speaking. Going over the expected due date is not a problem (see, when are you overdue?), and a great many of babies seems to be too comfy in there to be born. But studies show that the chance of stillbirth goes up drastically at the 42 week mark, making a compelling case for induction of labor at that time, or even a few days before.

Many moms will find that they get tons of irritating calls asking them whether their baby is born yet around the due date. If you want to avoid this, it's a good idea to avoid giving your friends and relatives an exact date, and instead saying that you are due in, for example, early January or late August.