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| |    Estimating Your Due Date by Marjorie Greenfield, M.D. reviewed by Marjorie Greenfield, M.D. What does a due date mean? While most women (and men) know what a due date is, they are often less certain of what it means and how it is determined. The old-fashioned term "estimated date of confinement" (or EDC) is simply another term for due date. In the years before we had our current technology, the only way to determine this date was by calculating 280 days (40 weeks) from the start of the last period. Keep in mind that conception does not actually occur until two weeks after the last menstrual period (LMP), which means that a full term pregnancy really lasts 38 weeks, not 40. Calculating your due date A simple method to identify your due date is to start with the first day of your last menstrual period. Subtract 3 months and add 7 days to this date. For example, if your last period began on June 7th, your due date would be approximately March 14th. This method is most accurate in determining your due date if you are certain of the date of your last period, and your cycles are approximately 28 days long. How many weeks pregnant am I? It can be confusing to be told that you are eight weeks pregnant when you know you only conceived six weeks ago. When we say an average pregnancy lasts 40 weeks, it means that it lasts 38 weeks from the date of conception, and 40 weeks from the last menstrual period. So you always get credit for two weeks before you even conceived! In general, any references to weeks in books, on ultrasounds, or at prenatal visits, will be based on a 40 week pregnancy. How many months pregnant am I? In pregnancy, another convention is to get credit for weeks or months only after they have been completed. At your due date you will have completed 40 weeks, or nine months. Unfortunately, since a month has four and a half weeks in it, calculating how many months you are can be perplexing. If you allow only four weeks for each month, you are ten months at your due date! The easiest way to figure out your month is to use your due date as the day you complete nine months, and then count backwards from there. For example, if your due date is November 17th, you will be eight months (and starting your ninth) on October 17th and seven months on September 17th. Because months can be confusing, medical people tend to count pregnancy off in weeks. A word to the wise There is an expression, "There's nothing worse than having the sun set on your due date and still being pregnant." While over 80 percent of babies are born within two weeks of their due date (before OR after), only around 5 percent are born on the due date. Some women decide not to tell anyone a precise due date. An old obstetrician I knew used to advise his patients to tell everyone that the due date was two weeks later than it really was. Many women find it very difficult when the pregnancy passes the awaited date, and are even more stressed when the phone calls and messages start to come in. Take heed and don't forget, as you contemplate your due date, that almost half of all babies arrive in the two weeks afterwards!
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