FAST FORWARDThe 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.
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.