Here's Why Your Due Date Is Really Just An Estimate, According To An Expert

When you first find out you're pregnant, the concept of waiting nine months feels like an eternity, and your due date feels like the holy grail. That number gets ingrained in your mind, and it's literally all you can think about. But it won't take long for you to learn that babies come into the world on their own terms and with their own agenda. So while it may be a good way to determine the approximate arrival of your baby, does your due date actually matter?

In an interview with Romper, Dr. Eva Martin, founder of Elm Tree Medical, says that only 4 percent of babies are born on their actual due date, and about 90 percent are born within two weeks of it. “Exactly why and how labor starts is one of the greatest mysteries of humanity," she says, noting that there truly isn't a definitive way to know the actual date of your baby’s arrival.

Even though due dates are not a guaranteed date of delivery, they are still important. Martin says that once you hit 41 weeks of pregnancy, doctors will offer an induction of labor to get things started to avoid any complications. She says that an overdue pregnancy, one that goes beyond 41 to 42 weeks, can increase health risks for your baby.

How is a due date calculated anyway? Most commonly, according to John Hopkins Medical Center, due dates are calculated as 40 weeks since the first day of your last menstrual period, with the assumption that your cycle is 28 days and that you ovulated mid-cycle. This method of calculation goes back 150 years, explained Women’s Health Care, and is based on Naegle’s Rule — a calculation method introduced by German obstetrician Franz Karl Naegele, who observed that a pregnancy usually lasts 280 days from the date of a woman’s last period.

While due dates are always just an estimate, Baby Center explained that most healthcare providers use a combination of the date of your last period and ultrasound imaging, which can give them a better idea of your baby’s size and gestational age. They can see how big or small your baby is to estimate how long it’s been since conception, and factor that into your due date.

While there is a low chance that your baby will be born on their due date, you can still have a time frame to look forward to and prepare for. The element of surprise and unpredictability your baby is giving you will continue well after delivery, so for now it’s best to sit back, relax, and plan ahead for your journey. (Better get used to your baby being in control now.)