Calculating Your Due Date

December 14, 2007

On April 7 of 1997, someone somewhere hit it right. That's the day, arithmetically speaking, that someone would have had to be conceived to be born on January 1st, 1998. This is based on the mother's last menstrual period beginning March 24 and ovulation occurring two weeks later, which is usually the way things go. The first baby of this New Year has no doubt won a slew of diapers, formula, and gift certificates. And next year's "first baby" awaits a cozy encounter next April to put his or her hat in the ring. Of course this is all fun with arithmetic.

"Term," or that point at which gestation is complete, spans a whole month's range. Labor is traditionally forty weeks after the last menstrual period, and term is anywhere from thirty-eight weeks to forty-two. So the question, "When is my baby due?" can only be answered with an approximation, because forty weeks, or term, is merely the center of a bell curve, 50% delivering on or before, and 50% delivering on or after the due date. The correct answer to when a baby is due is the due date give or take a couple of weeks.

Yes, there are those early risers that like to come early, pushing themselves into the world as premature babies. For this situation, we can go back even two more weeks to thirty-six. At thirty-six weeks the lungs usually reach maturity, but we obstetricians feel a whole lot better about thirty-seven if you want to know the truth. It's all arithmetic. The time-honored formula of subtracting three months and then adding seven days to the start of the last period still determines the official due date. Whether the baby actually has a birthday on that day may be nothing more than a romantic notion to the parents, but it's a temporal landmark for the obstetrician. A due date becomes extremely important when it is used to determine the severity of preterm labor--whether a baby should be allowed to deliver with a reliable degree of safety or would another week or two guarantee mature lungs. In other words, we don't care if a baby is born at thirty-eight, thirty-nine, or forty-one weeks. But we surely need to know when forty weeks is so that we can be correct in stopping the labor of a baby who is only thirty-three weeks. The importance of the due date to the obstetrician is emphasized when the baby chooses a date remote from this day.

This includes after, as well. The tail end of the term range is forty-two weeks, or two weeks after the official due date. After that, the placenta (the afterbirth responsible for nutrition and oxygen to the baby) begins dying, but the baby keeps growing (about a half a pound a week). Bigger needs for the baby clash with decreased means of delivering support by the placenta, and at some point there's going to be a crash. Most obstetricians draw the line at forty-two weeks, feeling letting a gestation go longer may include unacceptable risks.

How To Figure out Your Due Date

Now that we have all of that straight, everyone move up two weeks. Let's face it, the baby doesn't start developing during the last menstrual period, but during the conception that follows about two weeks later. So if term is forty weeks after the beginning of the last menstrual period, the baby really develops only during the thirty-eight weeks after conception. Although it would be more accurate to time gestation based on a thirty-eight week span, most women find it difficult to tell their doctors on that first visit when their last ovulation was; but they usually can report the time of their last period. In the past, when doctors themselves didn't fully understand the timing of ovulation as related to periods, this forty-week business started and through the sheer force of traditional convention stands solidly as the standard everyone uses.

Would you like to confuse your obstetrician? Just ask how many months pregnant you are. Forty weeks make up ten perfect four-week months. But applied to the calendar, with its months are a mixture of thirty, thirty-one, and even twenty-eight days, the forty weeks of a term gestation go only nine months. So halfway through a pregnancy is twenty weeks--five perfect months, but four and a half calendar months. One day we'll do this with logarithms.

And now that class is over, I raise a New Year's toast to those who partied hard New Year's Eve, because as an obstetrician I eagerly await next October 8. Give or take two weeks. See you then.

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