Due Dates: When is Baby Really Ready?

Every first-time mom anxiously awaits their due date. They hit the 20-week mark in the pregnancy and think, "Halfway done!" Plans are made for the days surrounding the due date, the calendar is marked, and everything seems to be focused on that all-important day. But where did that idea even come from, and is it accurate?


"The 40 week due date is based upon Naegele's Rule. This theory was originated by Harmanni Boerhaave, a botanist who in 1744 came up with a method of calculating the EDD based upon evidence in the Bible that human gestation lasts approximately 10 lunar months. The formula was publicized around 1812 by German obstetrician Franz Naegele and since has become the accepted norm for calculating the due date. There is one glaring flaw in Naegele's rule. Strictly speaking, a lunar (or synodic - from new moon to new moon) month is actually 29.53 days, which makes 10 lunar months roughly 295 days, a full 15 days longer than the 280 days gestation we've been lead to believe is average. In fact, if left alone, 50-80% of mothers will gestate beyond 40 weeks."

So here we are, 200-odd years later, basing our method of calculating babies' due dates on a theory formed by an OB that was inspired by another theory from a botanist nearly 70 years prior. Never mind that it's been proven that the majority of moms deliver later (only about 3-4% of babies come on their exact due date), most first-time moms deliver around 41 weeks, and the ACOG guidelines -based on modern evidence- state that a pregnancy is only considered post-term if it lasts longer than 42 weeks. Your doctor will still, more than likely, give your due date based on your LMP (last menstrual period), calculate your due date for 280 days from your LMP, and that is when you/they will expect baby to arrive.

But is that method of calculation accurate, either? Nope. Women's cycles aren't all exactly the same, and the LMP method assumes that your cycles are 28 days long, and that you ovulate on day 14. This is simply not true for all women. Only an estimated 10% of women fit into the "perfect" 28-day model, so why are all women's due dates calculated this way? It can lead to inaccurate due dates, off by 1-4 weeks depending on your individual cycle. This can lead to an induction happening because the doctors think you're past 41-42 weeks, when in reality, due to your cycle length, you're only 37-40 weeks, and as ACOG does not recommend inducing prior to 39 weeks unless medically indicated, you run the risk of your baby being what is called "early term," which can bring with it many complications (jaundice, difficulty breathing, NICU admission, etc.).

A healthy pregnancy is classified as lasting anywhere from 38-42 weeks, so think of your "due date" as the average time your baby might come. But you are not, I repeat not, "overdue" until you are 42 weeks or beyond. If you pass that long-anticipated 40 week mark, don't panic. It's perfectly normal, and no cause for concern in and of itself. There are plenty of things your OB/midwife can do to make sure you and baby are still doing fine (non-stress tests, ultrasounds to check that the placenta is still functioning, etc.), but simply being in the 40-42 week window is not an automatic issue. So think of your due date as more of a due month, and try to relax. If you and your baby are healthy and strong, there's no reason not to wait it out, even past 42 weeks! All babies are different, and so are their moms.