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

Weeks or 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.