Your Due Date


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. 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

The due date is referred to as the estimated date of confinement (EDC). This is a throwback to the days when a woman was “confined” to bed and home for a period of time after birth until convalescence was complete. It was a term contemporary with the “lying-in” that meant the same thing.

By the time you’re “late” for your period, that period which will never come, you are already two weeks past the time of conception. According to the usual way pregnancy is calculated, you are considered four weeks pregnant. At least we obstetricians call it four weeks, because we count from the last regular menstrual period. But since no one is pregnant before conception, this is a source of confusion on every pregnancy until the way we count is explained.

The normal human gestation is about 280 days. Divided into perfect four-week months, this comes out to ten perfect months. But Julius Caesar and Pope Gregory XIII, the architects of the modern calendar, were thinking of anything but gestation when, as the sing-song goes, “Thirty days hath September, April, June, and November.” (It goes without saying that all the rest but one have thirty-one….) If an obstetrician had designed the calendar, every month would have 28 days (“and no more”), comprising 13 perfect four-week months per year. Actually, this “OB” year would be just a little short, the real solar year being 13 perfect months and about one and a quarter day.

I know I could figure out something to do with that extra one day, five hours and forty-nine minutes.

The Gregorian calendar and the OB calendar don’t jibe. The OB calendar uses perfect four-week months — lunar months; the Gregorian calendar uses months of thirty, thirty-one, and even twenty-eight and twenty-nine days. The forty weeks of pregnancy take up nine months according to the Gregorian calendar.

It is no coincidence that the average menstrual cycle lasts twenty-eight days. These intervals have been called “lunar” months in the past, coinciding with the repetitive phases of the moon. The word “menses,” from which is derived “menstrual” cycle, is a Latin connection with the word for moon. Periods historically coming every lunar month only serve to teach us that we are so very implanted into this world, its spin, and its satellite. Over the long spans of evolution, the exact relationships have been lost, but it makes sense that the pineal gland in your head, a vestige of a third eye that in lower animals responds to light and helps with circadian rhythm and which is involved with pigmentary changes affected by estrogen and progesterone, is linked to the periodic light of the moon and the mysteriously related menses.

Each time a calendar month spills over the perfect twenty-eight days, the perfect gestation of ten perfect months gets out of sync with it. The two days here and three days there eat into that tenth perfect month, so that by the printed calendar of today a pregnancy lasts nine months.

OB Due Date Arithmetic – Don’t Forget to Carry the One

On April 7, someone somewhere hit it right romantically. That’s the day, arithmetically speaking, that someone would have had to been 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 this is all fun with the arithmetic.

“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.


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.”

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.

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 would another week or two 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.

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.

Let’s face it; your baby doesn’t start developing during the last menstrual period, but during the conception that follows about two weeks later. So if term is 40 weeks after the beginning of your last menstrual period, your baby really develops only during the 38 weeks after conception. In the past, when doctors themselves didn’t fully understand the timing of ovulation as related to periods, this 40-week business started and through the sheer force of traditional convention stands solidly as the standard everyone uses. Ovulation isn’t so apparent. The 40-week method remains because a period is an outward sign a woman can report to her doctor. Although it would be more accurate to time gestation based on a 38-week span after conception, you would probably find it difficult to tell your doctor on that first visit when you ovulated; but you can usually report your last period with reliability.

Would you like to confuse your obstetrician? Just ask how many months pregnant you are. Even though the 40 weeks of pregnancy make up ten perfect four-week months, applied to the uneven printed calendar the 40 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.

I know what happened the previous April 7 when I deliver a baby on New Year’s, and while I’m working I also realize that there are others celebrating the holiday which will see me in action the following September 25th. Give or take two weeks. For me, class is never over.

Article Posted 6 years Ago

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