FAST FORWARDTerm 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.