The Myth of the Due Date, Explained

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An Austin Area Birthing Center mom recently wrote to us: “Help! I have three different due dates, and I don’t know which one is right!”

Here’s how that can happen. The average pregnancy lasts about 280 days or 40 weeks from the first day of the last menstrual period. But babies grow at different rates: some may arrive 2-3 weeks before the predicted due date, and some 2-3 weeks after. The baby was conceived after the menstrual cycle began—but the mystery is exactly when fertilization occurred.

Calculating the due date is important for a few reasons. One is to prevent induction of labor for a baby more than two weeks late (post-term). Another is to attempt to stop preterm labor (before 37 weeks). Finally, many tests and procedures are based upon the appropriate age of the baby.

Before ultrasound, one the best ways to calculate a due date was Naegele’s rule. Pregnancy calculators are based upon it. Count back three months from the first day of the last menstrual period, and add 7 days. For example, if the first day of the last menstrual period was February 20, then the estimated due date would be November 27.

However, Naegele’s rule and pregnancy calculators are based on a 28-day cycle and assume that ovulation occurred on day 14. For many women, cycles can be much shorter or longer, and ovulation generally tends to occur later rather than earlier. Sperm can also live for up to five days in the woman’s body prior to fertilization. So even when the date of intercourse is known, that is not necessarily the date conception occurred.

As ultrasound has become more sophisticated, pregnancies can be identified earlier.  Due dates can be calculated more specifically when performed early in the pregnancy because new babies begin growing at about the same rate. Once the pregnancy is more than 20 weeks, fetal growth can vary greatly, and by the third trimester, using an ultrasound for dating can be off by 3 or more weeks.

In women with long or irregular cycles, or if pregnancy occurred within three months of using hormonal birth control, then using the “last menstrual period” calculation won’t be as accurate as a first trimester ultrasound for dating. Even with regular 28-day cycles and a certain knowledge of your last period, women who underwent first trimester ultrasound for dating more likely delivered within 7 days of the ultrasound estimated due date than the due date calculated from their period.

In general, if an ultrasound was performed before the 14th week, and the estimated due date is 5-7 days different from the last menstrual period calculation, the ultrasound due date should be used. From 14-20 weeks, if the two due dates are as much as 10-14 days apart, the ultrasound date should be used.

It is really not uncommon to have been told several due dates. The important thing to know is how the due dates were calculated, and whether they are within a few days of each other.  Always talk with your provider about the due date and let them know as much as possible about your cycle frequency, when you think you conceived, and if you’ve had an ultrasound prior to visiting them for the first time. The rest is up to Mother Nature!