Pregnancy Math: 20 Weeks by Whose Count?

Most of the recent abortion debate in the US has revolved around banning abortions after the 20th week. What many people (including lawmakers in at least one state) don’t realize is that there are two ways of dating a pregnancy – gestational age and embryonic age. Gestational age refers to the number of weeks the woman (or man, orother uterus-bearing individual regardless of gender identity) has been pregnant, while embryonic age refers to the actual age of the embryo/fetus. Yes, those are different numbers. Confused yet?

Many pregnant women may not know the exact date of conception unless they were actively tracking ovulation, only had (unprotected) sex once in the timeframe when they could have ovulated, or underwent fertility treatments such as IVF or artificial insemination. Therefore, most pregnancies are actually measured from the first day of the last menstrual period (LMP) and the due date is calculated by adding 280 days (40 weeks) to that date. If the exact conception date is known, the due date can instead be calculated by adding 266 days (38 weeks), since the first method assumes a 28-day cycle where ovulation occurs 14 days after menstruation begins. In some cases when neither the LMP nor conception date is known, the pregnancy can be dated by examining the fetus via sonogram. This online calculator is the most comprehensive I’ve seen; it gives five different options to figure out the approximate due date.

So yes, this does mean that when we talk about a typical 40 week long pregnancy, the woman isn’t actually carrying a child for the first two weeks or so of the pregnancy. Bizarre, right?

Most of the laws regulating abortions are written with the embryonic age in mind, but this isn’t always the case. In April 2012, Arizona passed a law (which was later struck down as unconstitutional) banning all abortions after 20 weeks counting from the LMP, meaning the fetus would only be about 18 weeks old. While other laws do refer to an embryonic age of 20 weeks, many women may not realize that they would still be able to obtain an abortion until they’re 22 weeks pregnant by the conventional reckoning. Many fetal abnormalities cannot be detected until the pregnancy is about 18-20 weeks along, which is when many obstetricians recommend checking the fetus’s development via sonogram. If a problem is suspected that could cause the pregnancy to be inviable, follow-up tests will need to be scheduled to confirm it in order for all parties involved to make an informed decision as to how to proceed. Laws that ban abortions at an arbitrary 20-week mark can force people to make hasty decisions, especially if there’s any delay in getting appointments or if they don’t realize the two-week difference between gestational and embryonic age.

By [E] Hillary

Hillary is a giant nerd and former Mathlete. She once read large swaths of "Why Evolution is True" and a geology book aloud to her infant daughter, in the hopes of a) instilling a love of science in her from a very young age and b) boring her to sleep. After escaping the wilds of Waco, Texas and spending the next decade in NYC, she currently lives in upstate New York, where she misses being able to get decent pizza and Chinese takeout delivered to her house. She lost on Jeopardy.

3 replies on “Pregnancy Math: 20 Weeks by Whose Count?”

I always wondered about using the LMP calculation on women whose periods are not regular due to PCOS, diet/exercise, stress, etc. I guess the sonogram method would be the next best measurement? It seems to me that there are too many “what if” variables in determining how far along a pregnancy is to be made into the rule of law (not that this should even be a law in the first place).

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