Why Your Due Date Is Almost Certainly Wrong

Photo: Francis Specker/CBS/Getty Images.
If you've been on #BeyWatch high alert for the past few weeks you are not alone. We know the Carter twins are imminent, we just don't know when they'll be born. But the truth is that even if Bey had publicly shared her due date, the chances of the twins arriving on that day are hilariously slim. In fact, over 90% of babies don't come on their given due date.
A huge part of the confusion is thanks to the wonky way we calculate those dates. As Science of Us explains, it's done with a complicated formula developed in the 1800s. The formula, called Naegele's rule, is designed to estimate the day you conceived your baby and calculate your due date from there. Essentially, your doctor counts forward one year from the first day of your last period, counts back three months, and counts forward again seven days.
That adds up to about 40 weeks after the first day of your last period. And if that doesn't seem imprecise enough, all of this is based on the assumption that everyone has a 28-day menstrual cycle and that everyone ovulates on the 14th day of that cycle (they don't). Plus, there's the fact that doctors sometimes change your due date when taking ultrasound results, your irregular periods, or later testing into account. We also know that first pregnancies are usually longer than subsequent ones, yet due dates are all calculated the same way. If you, like Bey, are carrying twins, you'll probably give birth earlier (usually around 37 weeks), but your due date calculations will be the same.
So, if you have a less than 10% chance of having your baby on your due date, when can you really expect to have your baby? Well, most people do give birth within two weeks of their given due date (partly because doctors will almost certainly induce labor if you're two weeks late), and the most common weeks to give birth are between weeks 39 and 40. That doesn't exactly help pin down one specific day, but that's about as exact as we can get.
Things change a bit if you had in vitro fertilisation, because you know your conception date for sure, which makes the rest of those calculations more accurate. And, of course, if you've scheduled an induction or a C-section you know exactly when baby is coming. For most of us, though, due dates provide an approximate window at best. So even if you're not awaiting a celebrity baby, it's probably a good idea not to stay too attached to a single day.