Why Are Due Dates Usually So Wrong?

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Photo: Mikolette/Getty Images

Dear Mona,

My wife and I are expecting our first child, and at our first appointment with her OB/GYN, we were told that only 5 percent of babies are born on their due date. Five percent!

It got me wondering: How accurate are due dates, anyway? What percentage of babies are born within a day of their due date? A week? When can I realistically expect this kid to arrive, in other words?

Thanks,

Matt, 32, Boston

Dear Matt,

A pregnancy, especially in the earliest weeks, is all about unpredictability, so it’s no wonder you’re seeking the sweet certainty of numbers right now. But it might surprise you to know that doctors still calculate the expected date of delivery using a method formulated at the start of the 19th century. The method, known as Naegele’s rule, reminds me a bit of the back-and-forth of the box step: You take the first day of a woman’s last menstrual period, add a year, subtract three months, and add seven days. Confusing, right? After those numerical acrobatics, you get a due date that’s about 280 days after the start of a woman’s last period, or roughly 9.2 months.

There are a couple of issues with this methodology, though. I don’t know about you, but I can barely remember what I had for lunch last Thursday, let alone the first day of my last period. I suspect that if a doctor asked me that, I’d just have to make my best guess. Also, Naegele’s rule is based on the assumption that women have a 28-day menstrual cycle, with ovulation on day 14, but women’s cycles range anywhere between 21 and 35 days. These issues might help explain why Naegele’s rule isn’t very accurate and, therefore, why due dates aren’t, either.

And that 280-day figure also isn’t exactly right. In 1990, a team of Swedish researchers looked at 427,581 births of singleton babies — that is, not twins or multiples. Each mother in this study was confident about the date of her last menstrual period, and each baby was delivered vaginally. Using data from 1976 to 1980, the researchers found that the average date of birth was 281 days after the first day of the woman’s last menstrual period. The median — that’s the middle value when all the data points are lined up — is higher, at 282 days. The mode — the most common result — was higher still, at 283 days. True, three days is technically not a very long time. But tell that to an expectant mother who has passed her supposed “due date.”

It seems like you’re also interested in variance — in other words, the probability of your wife giving birth in any given week of her pregnancy. According to a Centers for Disease Control and Prevention report released in December, there’s a 59 percent chance that your child will be born at full term, Matt. If I knew the age or race of your wife — or whether this is her first child — I might give a slightly different answer, as these are all factors that can affect the timing of a birth. Research has shown, for example, that women under the age of 19 or over the age of 34, black women, and those who have already given birth to at least one child are likely to have shorter pregnancies.

Births by term

Pre Term Under 27 weeks 0.69% 28–31 weeks 0.91% 32–33 weeks 1.15% 34–36 weeks 6.82% Early Term 37–38 weeks 24.76% Full Term 39–40 weeks 58.72% Late Term 41 weeks 6.53% Post Term 42 weeks or more 0.42%

But that’s just the data for one year. If you take a wider view, it becomes clear that childbirth trends have changed quite a lot in the past several decades. The percentage of births that happen at 42 weeks or more has dropped by half since 1990, and the percentage at 40 or 41 weeks has dropped off considerably, too. That can largely be explained by another change that’s happened in the U.S. over that same period: C-sections and labor inductions have become much more common.

Births by week of pregnancy

Hover over or tap the points on the graph for more detail.

Under 37 weeks 37–38 weeks 39 weeks 40–41 weeks 42 weeks or more
19902000200520062007200820102011201220130%10%20%30%40%

Those two changes also explain something else that really surprised me about the data. Babies are much more likely to be born in America on a weekday rather than a weekend. In 2014, for instance, 12,630 births occurred on average on any given Wednesday compared to 7,371 on a Sunday. Far fewer of those weekend births were C-sections; doctors like their weekends, too, so they’re much more likely to schedule nonurgent procedures for during the week.

And yet, that doesn’t explain the annual variance in the data on American childbirth: July was the biggest month for births in 2014.

Births by day of the week

Average # of Births
7,000 10,000 13,000
11,805
12,630
12,155
12,112
12,042
8,344
7,371
Monday Tuesday Wednesday Thursday Friday Saturday Sunday
Day of the week

In 2014, Micaela Martinez-Bakker and Kevin Bakker, both of the University of Michigan, published an incredibly comprehensive analysis of birth dates in the Proceedings of the Royal Society. They looked at 78 years of monthly birth data in the U.S. and found that northern states have their peak birth months earlier in the year than southern states. When they looked at more than 200 other countries, they found that the same pattern held true: Those in the northern hemisphere tended to give birth earlier in the year. There are no bulletproof explanations for this. But an MIT paper in 2013 suggested that births that are seasonal are more likely to be ones where the woman planned to get pregnant. Maybe women are trying to time their babies according to the school year, when they’d like to take time off work; perhaps the quieter summer months are seen as a better time to take maternity leave. Or maybe taking that time off work during the summer is just more fun. These are just guesses, though.

There’s one more thing that tends to get looked over, despite all of this data, plus the sophistication of modern scanning equipment: the simple fact that “not all babies are the same,” said Emily Oster, an economist at Brown University. Oster is the author of Expecting Better, which is all about pregnancy misconceptions. She points out that if a scan shows a slightly larger baby than its due date would imply, maybe that is indeed a sign that the due date is off. Then again, a larger-than-average baby might be just that — a larger-than-average baby. Oster’s general advice is to put down the calculator and the calendar and let the baby do its thing. Babies don’t care much for data — they’ll come when they’re ready.

Whenever the birth takes place, I hope you and your partner have a delivery with as little uncertainty as possible.

Hope the numbers help,

Mona

Mona Chalabi is data editor at The Guardian U.S. You can send her questions for her "Dear Mona" column at mona@monachalabi.com or on Twitter.