Here's why '9 months of pregnancy' is a vexing myth

When my wife became pregnant with our first child, I believed women were supposed to carry a baby for nine months before giving birth.But I soon learned this was a myth.
No matter how you estimate the duration of a "typical" pregnancy - using the latest scientific data on pregnancies or converting months into weeks or days - nine months misses the mark.
Just 4% of pregnant women deliver a baby after 40 weeks, which is a number used interchangeably with nine months.
But even nine months does not equal 40 weeks, or 280 days - another number you'll hear from obstetricians, government agencies, and other sources of information you encounter during a pregnancy.
That's because months vary in days. Averaging 365 days over 12 months, your average month lasts 30 days and 10 hours. So 280 days is actually nine months plus nearly a week.
While this is a minor quibble, the core figure of 280 days is also suspect.
The number first appeared in a German manual for midwives published in 1836, and it's an estimate of when a woman ovulates to release an egg all the way through to birth. Because ovulation can be notoriously difficult to detect, however, researchers at the NIH in 2013 decided to check this number with the latest scientific tools.
The NIH researchers followed 125 women, starting with a sensitive chemical test for ovulation all the way through to the birth of one child. (Another mark against "nine months": A vast majority of twins and other multiples are born in the seventh or eighth month of pregnancy.)
The researchers discovered that the median pregnancy was not 280 days, but 268 days - and that was after excluding pre-term or post-term babies and accounting for a woman's weight, alcohol use, sex during pregnancy, and other factors.
This means a "typical" pregnancy likely lasts 38 weeks and 2 days - or 8 months, 24 days, and 16 hours.

The NIH study's real kicker, however, is how much the length of a traditional pregnancy can vary: by a whopping 37 days, or five weeks, spanning before and after the 268-day mark.
The reality is that every woman's pregnancy is different, as science writers Tara Haelle and Emily Willingham make abundantly clear in their meticulously researched book, "The Informed Parent: A Science-Based Resource for Your Child's First Four Years".
And if you aren't convinced this actuarial nitpicking matters, consider the fact that some doctors can and do rush pregnant women into cesarean sections because of timing concerns, and the procedure - while typically very safe - not only costs thousands of dollars more than a vaginal birth, but also increases many kinds of risks to babies and their moms.
All of that said, the NIH researchers did not go so far as to make a clinical recommendation; after all, 125 women is a small sample size.
Yet it stands to reason that freely passing around a nice, round, and imprecise number could have some real and unfortunate consequences.

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