Fritz and son
While I'm perpetually showing up to places right on time—okay, I'm usually late—my wife is always early. But as she neared her pregnancy due date, it became clear that one of the greatest anxieties of childbirth comes at the very end. Will the labor start naturally on time, or will the baby be so late that induction or Caesarean section is necessary?

If the goal is a drug-free birth—as ours was—being told you have to be induced could force a dramatic reimagining of your birth plan. If the goal is a vaginal delivery, being told you will need a Caesarean could be heart-wrenching. Yet being past your due date is not only a common anxiety for pregnant women, it's an incredibly common situation.

The 40-week—or nine-month—pregnancy feels like an indelible part of our culture. Store shelves are lined with books that reference it, movies have been named for it, and serious medical decisions are made because of it.

The concept of a 40-week pregnancy was popularized nearly 200 years ago by Franz Naegele, director of a maternity hospital in Germany and author of a textbook for midwives. In a method called Naegele's Rule, he calculated the date of birth by adding 280 days to the date of last menstrual period.

Naegele borrowed this idea of a 280-day gestation from the 18th-century Dutch physician and botanist Herman Boerhaave, who created the modern teaching hospital. Boerhaave based his estimate of a 280-day gestation on evidence from the Bible that pregnancy lasts 10 lunar months.

There's just one snag in Boerhaave's theory and Naegele's Rule—most pregnancies are not exactly 280 days. Only about 5 percent of women give birth on their due date.

In The Numbers Game, journalist Michael Blastland and economist Andrew Dilnot explain how common misunderstandings of statistics affect our perception of the world. They use the 280-day pregnancy to explain the essential problem with averages. "Two facts about pregnancy suggest that the simple average will be misleading. First, some mothers give birth prematurely. Second, almost no one is allowed to go more than two weeks beyond the due date before being induced," they write. "The effect of this imbalance—we count very early births but prevent the very late ones—is to produce a lower average than if nature were left to its own devices."

Blastland and Dilnot cite a study in Sweden that found half of all babies were born by the 282nd day of pregnancy, while Day 283 saw more births than any other single day. "If most women have not had their baby until they are least two days overdue," they write, "and women are more likely to be three days overdue than anything else, it invites an obvious question: Are they really overdue?"

Meanwhile, other studies have shown first-time mothers' pregnancies tend to last even longer—288 days. Whether she is truly "overdue" or not, going past 280 days ramps up the stress for expectant parents. Punctuality is a virtue in our society. Show up 15 minutes late to class, and you get sent to the principal's office. Clock in an hour late for work, and you might get fired.

For overdue mothers, the threat is medical. Some doctors start talking to patients about induction or Caesarean section just a day or two after the due date. The Swedish survey suggests some pregnant women might find themselves being induced before they're even actually overdue.

If there is no life-threatening medical problem, having the flexibility to go two weeks past the due date could make a huge difference. According to, around 80 percent of babies are born between the 38- and 42-week marks.

Medical science doesn't know exactly what causes labor. Some think it starts when the mother's body releases oxytocin. In fact, the drug Pitocin, which is used to induce labor, is synthetic oxytocin. A few new theories speculate that the baby's body starts labor by releasing hormones. While what causes labor remains a mystery, there are plenty of folk remedies that promise to speed it up. These include taking a long walk, eating spicy food and having sex—which sounds like the kind of date that could have caused the pregnancy in the first place!

To preempt her own worries about a punctual birth, my wife always told interested parties her due date, but was sure to add that she had no expectation of meeting it. And she was right—she blew right past it.

Then on the 286th day of her pregnancy—officially six days late, yet two days earlier than the average for first-time moms—she went into labor after watching an episode of Mad Men. A few weeks later, halfway through the opening credits of another Mad Men episode, one of our friends went into labor too. I think we may have something here. If you know a pregnant woman who is at 280 days, tell her to wait three days and then turn on her TV.

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