The Truth About Due Dates

Only 4% of pregnant people give birth on their due date. Read on to find out how doctors estimate when your baby will make their appearance.

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While most babies are born within two weeks of their due date, very few arrive "on time." In fact, only 4% of pregnant people give birth on their due date. Unsurprisingly, I wasn't one of them. Both of my children arrived before or after their given due dates. My oldest was a week late. My daughter was a week early. And that got me thinking: What's the point of estimating an arrival date anyway? Why are they called "due dates" if they aren't accurate or exact?

Here's everything you need to know about due dates, from how they are calculated to what factors impact them—and when your little bundle of joy will actually arrive.

What Are Due Dates?

Due dates or estimated delivery dates (EDD) are just that, an estimate of when your baby will be born. While the number of babies delivered on their due date is few and far between, it does happen. Believe it or not, there is an imperfect science to calculating your baby's due date.

For the most accurate estimate, you'd want to calculate your due date from the day of conception, but it's not always easy or possible to pinpoint the exact day fertilization happened. Instead, a more general and accessible guideline to follow is to count 40 weeks from the date of your last menstrual period (LMP)—though this method assumes you ovulated on day 14 (based on a 28-day cycle), which may not be the case.

"Knowing how far along you are, makes it easier for your obstetrician to see that your baby is growing properly," explains OB-GYN Joanne Stone, MD, co-author of Pregnancy for Dummies.

Rather than breaking out the calendar to count, you can also use a due date calculator, such as the one below, to estimate your due date.

What Is the Best Way to Figure Out My Due Date?

If you can't remember the first day of your last period or aren't totally sure when conception occurred, never fear: In addition to calculating your due date using information about your last cycle, your health care provider may conduct a transvaginal ultrasound to get an estimate on how far along you are (and by extension, when you're "due").

Although this ultrasound is conducted internally, it is safe for you and your baby. It's usually used during the first trimester when external abdominal ultrasound isn't as effective. A happy bonus: This ultrasound might be the first time you get to see your little one's earliest cardiac activity.

Factors That Can Affect When You Deliver

Your little one might start testing your patience before they even arrive. If your due date has passed and there are no signs of labor, don't worry just yet. Most pregnant people (about 80%) deliver sometime between 37 and 42 weeks, and about 11% deliver prematurely.

While there aren't any hard and fast rules as to why some people deliver before their due dates or why others deliver later, there are a few things these groups tend to have in common:

  • Multiples. You're more likely to be part of the crowd that delivers before your due date if you're carrying multiples (twins, triplets, or more).
  • Age. Your age can impact the duration of your pregnancy. Those who are 35 and older, for example, are more likely to deliver prematurely.
  • Weight. Both the weight of the pregnant person and the fetus can affect your delivery date.

Chronic health conditions can also affect your due date. High blood pressure, for example, can cause preeclampsia, which can lead to the decision to deliver a baby before their due date.

On the flip side, you may also deliver after your due date. As annoying as it can be (because sometimes pregnancy just feels like it will never end), there isn't too much risk associated with going overdue. However, expect a few things to change. For example, beginning at 40 or 41 weeks, expect to undergo weekly, then twice-weekly, non-stress tests. Your provider might also discuss scheduling an induction if labor doesn't start on its own.

What Is the Most Common Week To Go Into Labor?

According to the American College of Obstetrics and Gynecology, the average length of a pregnancy is 241 days or 40 weeks long and the most common time that pregnant people will spontaneously go into labor is between weeks 38 and 41.

Researchers have now identified that labor may actually begin two to four weeks before any signs of labor become apparent. According to a study from the Standford University of Medicine, a combination of signals from the immune system, clotting factors, and hormones shift a pregnancy from the "progressing pregnancy" stage to the "pre-labor" stage, and it can be detected using a blood test that looks at immune, hormone, and blood clotting indicators. This research may mean that doctors could potentially test a pregnant person to see if they are in pre-labor.

Once you go into labor, you might be wondering when you'll give birth. Well, research has identified when that's most likely, too. According to one study, most spontaneous births happen between 1:00 and 6:59 a.m., with a peak at 4:00 a.m.

Key Takeaways

So what's in a due date? Well, this estimate of when your baby will arrive is typically based on the date of your last period before conceiving, but when you will actually give birth will depend on many other factors. If you have questions about your estimated due date, run them by your health care provider. Whether your baby comes before, on, or after your due date, they'll be in your arms before you know it.

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Sources
Parents uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Patient education: Postterm pregnancy (Beyond the Basics). UpToDate. 2024.

  2. Preterm Birth. World Health Organization. 2023.

  3. Gestational Hypertension and Preeclampsia. American College of Obstetricians and Gynecologists. 2020.

  4. Prenatal Nonstress Test. StatPearls. 2023.

  5. How To Tell When Labor Begins. American College of Obstetricians and Gynecologists. 2023.

  6. Integrated Trajectories of the Maternal Metabolome, Proteome, and Immunome Predict Labor Onset. Science Translational Medicine. 2021.

  7. Timing of Singleton Births by Onset of Labour and Mode of Birth in NHS Maternity Units in England, 2005–2014: A Study of Linked Birth Registration, Birth Notification, and Hospital Episode Data. PlosOne. 2018.

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