The Truth About Due Dates

While most children are born within two weeks of their due date, very few arrive "on time." According to data from the Perinatal Institute, only 4% of people give birth on their estimated date—and I wasn't one of them. Both of my children arrived before (or after) I anticipated. My oldest was a week late. My daughter a week early. And that got me thinking: what's the point of estimating a 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 arrives.

What Are Due Dates?

Due dates or, estimated delivery dates (EDD) are just that, an estimate of when your baby will make their appearance. While it seems like the number of babies delivered on time are few and far between, it does happen. Believe it or not, there is a science of sorts to calculating your pregnancy due date.

Technically, you could calculate your due date from the day of conception, but it's not always easy to pinpoint the exact day fertilization happened. Instead, a more general guideline to follow is to count 40 weeks from the date of your last period.

"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, M.D., co-author of Pregnancy for Dummies.

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 your last cycle, your doctor may conduct a transvaginal ultrasound.

Although this ultrasound is conducted internally, it is safe for your baby. It's usually used during the first trimester and might be the first time you get to see your little one's heartbeat.

What Factors Affect Your Due Date—and Will My Baby Come Early?

Your little one might start testing your patience before they arrive. Even if your due date has passed, if your baby hasn't dropped yet, or there haven't been any changes to your cervix, hold tight until your next doctor's appointment. But don't worry, you certainly aren't alone. 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 have in common:

• Multiples. You're more likely to be part of the crowd that delivers before your due date if you're carrying 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 delivery prematurely.
• Weight. Both the weight of the expectant parent and the unborn child can affect your delivery date.

Chronic conditions can also affect your due date. High blood pressure, for example, can cause preeclampsia. This can lead to inductions and/or delivering 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. However, expect a few things to change. Beginning at 40 or 41 weeks, expect to undergo weekly, then twice-weekly, non-stress tests.

The Bottom Line

So what's in a due date? Well, this estimate of when your baby will arrive is based on the timing of your last period, your overall health, and can fluctuate. If you ever have questions or concerns, run them by your provider. Until then, enjoy the magic of pregnancy—or, at the very least, enjoy uninterrupted naps. Whether your baby comes before, on, or after your due date, they'll be in your arms before you know it.