What Due Dates Actually Mean

Pregnancy due dates can be tricky things: On one hand, they're an exciting milestone that many pregnant people are eager to reach. On the other, they can quickly become a misleading "deadline" that may cause more stress and confusion than excitement.

Doctors call due dates EDDs (which stands for "estimated delivery date") for good reason: They are just an estimate that can be off by as much as two weeks, explains the American College of Obstetricians and Gynecologists (ACOG).

Read on to learn about how accurate due dates are and when first-time parents actually give birth.

How Accurate Are Due Dates?

Very few people actually give birth on their due date. In fact, a 2013 study out of Australia found that only 5% of people give birth on their due dates, and recent developments such as inductions, C-sections, and more complex pregnancies have most likely decreased that number even more. The good news is that while you may not give birth exactly on your due date, your chances of giving birth to a full-term baby are good, barring any medical complications.

According to data from the Centers for Disease Control and Prevention (CDC), the majority of babies in the U.S. are born full-term, which means anytime between 37 and 42 weeks of pregnancy. Just over 10% of babies are born preterm, which means before 37 weeks. Very few babies are born "late-term," which the ACOG defines as over 42 weeks of pregnancy.

How Due Dates Are Calculated

The ACOG explains that traditionally, due dates have been calculated by adding nine months and seven days (or 280 days) to the first day of the pregnant person's last menstrual period (LMP). This calculation assumes that the person ovulated two weeks after getting their last period or on day 14 of their cycle.

While it's widely used, this method is not very accurate and doesn't account for things like menstrual cycle irregularities, ovulation being unpredictable, how long implantation can actually take, and of course, human error.

Of course, when the date of conception is known such as with in vitro fertilization (IVF), the estimated due date can be calculated by adding 266 days to that date rather than the first day of the LMP. This estimated due date will typically be a lot more accurate than a due date calculated using the LMP.

Nowadays, the ACOG considers a first-trimester ultrasound (up to right before 14 weeks of gestation) to be the most accurate way to predict a pregnant person's due date if the date of conception isn't known. In fact, if you get a first-trimester ultrasound, your doctor may adjust your initial due date depending on what the ultrasound shows.

The South African study also found that women who used both their last menstrual date and a smartphone app to track their cycles were able to predict their due dates more closely to their actual date of delivery than women who used just their period date alone. Talk to your doctor about using any cycle data you have to help predict your due date.

Average Week First-Time Parents Give Birth

Wondering about the average week of pregnancy that first-time parents give birth? Research suggests that about half of people giving birth for the first time give birth by 40 weeks and 5 days. This finding suggests that when left alone to go into labor without intervention, first-time pregnancies often last longer than the 40-week estimate used when calculating due dates.

Other research highlights the variability of birth dates compared to estimated due dates. The Australian study also found that 66% of pregnant people gave birth within 7 days of their estimated due dates. And a more recent study of 325 women out of South Africa also found that 88.3% of the women delivered within two weeks of their due dates.

That said, what's average has little influence on when an individual will give birth because there are many factors that go into determining when a baby is born, including medical interventions like inductions and planned C-sections.

How to Get Through the Last Weeks of Pregnancy

While you count down the long weeks and months to a due date that may or may not be accurate, you may experience some frustration, which is very normal. It's not always easy to get through those last weeks of pregnancy, especially if your due date has come and gone and you're incredibly anxious to deliver.

Getting through those last weeks is as much a mental game as it is a physical one, and how you think about your due date may help. Consider the following scenarios.

Two friends are both due at the end of August. Both are first-timers with uncomplicated, healthy pregnancies. They're both eager to meet their babies, but one is counting down the days to their due date like they're going on a cruise. "Twenty seven more days!” they write in an Instagram post.

The date is so embedded in the first friend's mind that frankly, others around them are a little worried. What will happen if they close in on their due date and there's no sign of labor? What if they sail right on past it? Will they be disappointed and impatient? Will they start doubting their body? They can't cancel the trip, but they might be tempted to change their travel plans.

The other friend, on the other hand, is taking a different approach. They know they're going on a "cruise" pretty soon. They're packed and ready to go, but they're not sure of the actual launch date. If labor doesn't start by their due date, they won't be disappointed, because they didn't have a firm date in their mind. They know to expect labor soon, but they're also realistic that there could be some unexpected delays or surprises along the way.

Now, everyone's experience of pregnancy is very different and there's no "right" or "wrong" way to approach your estimated delivery date, but in considering the two approaches, it could be argued that a more relaxed and flexible approach might help you get to the finish line a little bit easier—and it certainly sets the stage for the unpredictable nature of parenthood.