Everything You Need To Know About Due Dates

Very few babies arrive "on time." In fact, only 4% of pregnant people give birth on their due date. It's common to go into labor a week (or more) early or late and have a perfectly healthy baby. And yet, at your first prenatal appointment, your health care provider will give you an exact date when your baby is expected to be born.

So how important are due dates? Here's everything you need to know from how these target "birth days" are calculated to the average length of pregnancy in the US (it may surprise you).

How Are Due Dates Calculated?

In the medical field, due dates are often called estimated delivery dates (EDD). It's a helpful term, since due dates are just that: an estimate of when your baby will be born. In truth, there is an imperfect science to calculating your baby's due date.

"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.

Methods for calculating due dates include counting forward from the date of your last menstrual period or ovulation, or having a physician-guided ultrasound.

Counting forward from your last menstrual period

A commonly used guideline to assess your due date is to count ahead 40 weeks from the first day of your last menstrual period (LMP).

However, this method assumes you ovulated on day 14 of a 28-day cycle. (Conception occurs within hours of when your body ovulates to release an egg to be fertilized.) In truth, research shows that only 12% of people have a 28-day menstrual cycle, and in that group, the most common day to ovulate is day 15.

Counting forward from ovulation

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.

If you are trying to conceive, you can use tools to find out when you are ovulating, which can help you zero in on the best time to have baby-making sex. Most pharmacies sell over-the-counter ovulation test strips, or you can use the ovulation calculator below.

If you are tracking your ovulation, that should give you a good sense of when you conceived. Count forward 38 weeks to determine your due date by using a conception date.

Ultrasound

If you can't remember the first day of your last period or aren't sure when you ovulated, never fear: Your health care provider may perform 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 to hear a fetal heartbeat.

Factors That Can Affect When You Deliver

Your little one might start testing your patience before they even arrive, with most babies arriving before or after the due date you've excitedly plugged into iCal or circled on your wall calendar.

According to the American College of Obstetricians (ACOG), babies born between 39 weeks and 40 weeks and six days have the lowest risk of experiencing complications during childbirth. But if you go into early labor or have yet to feel a contraction once your due date has just passed, don't panic. What's important is that you keep in close touch with your health care provider through the course of your pregnancy.

You may be more likely to go into labor before your due date if you:

• Are carrying multiples
• Have anatomic differences, like a shorter cervix
• Are under age 18 or older than age 35
• Have certain infections
• Have high blood pressure or diabetes

You may be more likely to deliver after your due date if you:

• Are giving birth for the first time
• Are over age 30
• Are clinically obese
• Were born post-term yourself

What Are the Most Common Weeks To Go Into Labor?

ACOG says that the average length of a pregnancy is 241 days (or 40 weeks) and the most common window for spontaneous labor is between weeks 38 and 41.

Once you go into labor, you might be wondering when you'll give birth. Well, research has identified when that's most likely, too. When giving birth at home, pregnant people in the US are most likely to deliver their babies between 1 a.m. and 6 p.m. However, if they're giving birth at a hospital, they're most likely to welcome their little ones between 8 a.m. and 5 p.m.