Ask Childbearing: Due Dates
Q: How likely am I to give birth on my due date?
A. Not very. In fact, only about 4% of babies arrive on their so-called ‘due date’.
So why do we call it a due date when it applies so rarely? The way we calculate the due date is a very old practice based on the assumptions of a German doctor called Naegele in 1812. He derived his theory from a biblical notion that women gestate for ten lunar months. He proposed calculating from the first day of the last menstrual period, adding one year, subtracting nine months, and then adding seven days to arrive at the ‘due date’.
This method has a few flaws. One, is that a lunar month actually has 29.5 days, not 28. This would move the due date to more than two weeks past his calculations, which, as any pregnant person will attest, is a significant oversight. Another flaw is that he assumes all women have twenty-eight day cycles, and ovulate on day fourteen. We know this is absurdly untrue, as cycles vary widely from person to person. Another flaw is that babies develop at their own individual timetable, which is very different for each baby. Some develop faster, and some slower. After all, we don’t all hit puberty on exactly the same birthday, either.
Babies can actually be full term anywhere from 37 – 42 weeks gestation, which is a five-week long window. Approximately 85% of babies are ready to be born within this time frame, which means that an additional 15% of babies are born even earlier or later.
But for some reason we still insist on using this archaic due date system, even though we know that only a tiny minority of babies will be born on their exact due date (4%). In fact, so seriously do we take these due dates, that we use misleading language about when the vast majority of babies are born. Forty-one weeks is not ‘overdue’—it falls exactly within normal range. But we use terms like ‘early’ and ‘late’ as if we believed the due date to be accurate.
Even ultrasound dating only gives a two-week range of accuracy, slightly better if done before twelve weeks, and worse if done later. The margin for error increases the more the baby grows, so that ultimately, the window isn’t much clearer than Naegele’s method.
So please don’t expect your baby to arrive on your so-called due date. Statistically, the majority of first-time mothers give birth a full eight days after their due date, while a smaller number give birth prior to the due date. The only truly unusual outcome is to give birth on the actual due date itself. So when it comes to expectations, I propose stepping back and viewing your estimated due date as more of a due month. Or even a due-season. There is no compelling reason to narrow our focus down to such a misleading single date. So how likely are you to give on your due date? Not very likely at all.
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