Due Date Arithmetic: Don't Forget to Carry the 1!

New Year's Baby

On April 7, someone somewhere hit it right romantically. That's the day, arithmetically speaking, that someone would have to be conceived to be born the following January 1st. 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 in regular cycles. The first baby of the last New Year has no doubt won a slew of diapers, a dash of formula, and a heap of gift certificates. And a cozy conception encounter in April will allow another contender to put his hat in the ring for the spot of the next coveted “New Year’s” baby. Of course, the fun is provided by arithmetic.

FAST FORWARD The due date is called the estimated day of confinement (EDC), because traditionally women were confined to bed, “lying in,” from the day of delivery to convalescence was complete. The LMP (last menstrual period) is the only observable event from which to calculate a pregnancy without the aid of ultrasound. EDC (LMP B 3 months) + 7 days

"Term", or that point at which gestation is complete, spans a whole nine month's range, according to the printed calendar. Labor is traditionally 40 weeks after the last menstrual period, but because of variations among us, term is anywhere from 38 to 42 weeks. So the question, "When is my baby due?" can only be answered with an approximation, because 40 weeks, or term, is merely in the middle of a bell curve, half delivering on or before, and half delivering on or after the due date. The correct answer to the question of when your baby is due is your due date -- give or take a couple of weeks.

Early Birds

Yes, there are those early birds that like to come early, pushing themselves into the world as premature babies if they come before 37 weeks. At 36 weeks the lungs usually reach maturity, but we obstetricians feel a whole lot better about the 37-week mark 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 your official due date. Whether your baby actually has a birthday on that day may be nothing more than a sentimental notion for you and your husband, but it's a necessary temporal landmark for the obstetrician.

Importance of Due Date

Whether your baby is born exactly on her due date or not is unimportant if the discrepancy between the calculated arrival and the actual arrival isn’t great. But your due date becomes extremely important when it is used to determine the severity of preterm labor - whether your baby should be allowed to deliver with a reliable degree of safety or whether another week or two would guarantee mature lungs. In other words, we don't care if your baby is born at 38, 39, or 41 weeks. But we surely need to know when 40 weeks is so that we can be correct in stopping your labor at 33 weeks. The importance of the due date to the obstetrician is emphasized when your baby chooses a date remote from the calculated due date.

FAST FORWARD Term is usually designated as the day forty weeks after the first day of the last regular menstrual period, but it’s not a due “date” because there is too much variation in the general population. Since term is anywhere from 38 to 42 weeks, it’s really a “due month.”

Worries about going beyond your due date apply as well. The tail end of the term range is 42 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 your baby keeps growing as much as a half-pound a week or more. Bigger needs for your baby clash with decreased means of delivering support by the placenta, and at some point there's going to be a shortfall and then a crash. Most obstetricians draw the line at 42 weeks, feeling letting a gestation go longer may include unacceptable risks. Lately there has been an academic push toward intolerance past forty-one weeks. There is considerable variation among doctors when the “post-dates” issue becomes a deal-breaker. I still use 42 weeks. Some, some brave ones, may even go to 43 or 44 weeks if they can determine fetal well being with accurate surveillance techniques.

Now that we have that straight, everyone move up two weeks. Pay attention, because there’s a final in nine months.