Time to ditch the due date?

What is the evidence behind the due date calculation that the medical model SO often holds on to as if it were a fact?

Well, it comes from “Naegele’s Rule”, which in turn was based on a calculation put forward by the botanist, Hermann Boerhaave, circa 1744 (you heard it, his expertise was in plants not human biology).

Boerhaave’s key reference point was the New Testament in the Bible, from which he derived that human gestation was approximately 10 lunar months. He studied 100 (white, European) women and found that 99 had their baby in the time found by counting one week after their last period plus 9 months gestation from that time. Therefore he concluded that his Biblical guesstimate was correct.

Circa 1806 a German obstetrician called Naegele took Boerhaave’s back of a fag packet calculations and finessed it, suggestion 280 days (or 40 weeks) from the first day of the last period as the accurate length of gestation, as the first day of the last menstruation is a more plausible point to keep track of.

That’s what we still use today. Cool huh? Except there are a number of flaws in this theory…

Lunar months actually range from about 27-29 days in length. It seems likely Boerhaave would have used a synodic (standardised) lunar month which is 29 days long. If so, his calculation would mean gestation was 10 days longer than the 40 weeks Naegele landed on. Boerehaave wasn’t clear on whether his calculation was from the first or the last day of the last menstrual period, so it’s basically not implausible that our mate Naegele robbed us of the guts of 3 weeks gestation.

That aside, Naegele’s rule won’t work for you if your cycles are longer than 28 days, or if you don’t ovulate on day 14. There are many good reasons why people simply don’t know the first day of their last period. If the foetus you are carrying grows at its own pace and not to a schedule it has never heard of, your due date probably won’t be accurate for you. We know that lots of factors such as ethnicity, parity, maternal height, foetal weight all affect length of gestation.

Knowing a little more about the calculations we’re given, how do you feel about your EDD? Are you confident in its accuracy?