Myth Monday: Due Dates


Welcome to the first installment of my new blog series: Myth Monday. We'll discuss a common pregnancy, labor, or birth myth and why it may or may not be supported by evidence. Today's topic: Due Dates!

When are you due?

This is often the first question you hear when you find out you're pregnant. It's honestly one of the first things I thought to myself when I got my first BFP (big fat positive) almost 9 years ago. If you're anything like me, you immediately started planning and mapping out the next 9 months right up until that day. This caused me to experience a lot of anxiety towards the end of my pregnancy, wondering what would happen if my baby wasn't born "on time." So, what does happen if your baby goes past your due date? And what is a due date, anyway?

Is my due date accurate?

The origin of our concept of due dates was introduced in the 1700's by a German obstetrician named Franz Naegele. Naegele's Rule estimates the date of delivery (EDD) by adding a year, subtracting three months, and adding seven days to the origin of gestational age. This results in a pregnancy lasting 280 days (or 40 weeks) from the first day of your last menstrual period. This is often referred to as the LMP (last menstrual period) method. So where did the 40 weeks come from? Well, Naegele observed pregnant women and concluded that pregnancies lasted, on average, 10 moon cycles. With about 4 weeks per moon cycle, this gives us 40 weeks. However, a moon cycle is not exactly 28 days. Naegele's Rule also assumes that every menstrual cycle is 28 days, and that most people get pregnant on day 14. This is not always the case.

Because of these variables, your doctor may determine your EDD with an early ultrasound, around 11-14 weeks. This method measures the fetus and compares the size to a reference group of pregnancies with known due dates, which determines the age of the fetus at the time of ultrasound. Early ultrasounds are more accurate than using your LMP, even if you are absolutely positive about your dates. You're almost four times more likely to go "overdue" if your due date is calculated by your last menstrual period vs. an early ultrasound.

Even with advances in technology, there is no way to determine your exact due date. Normal pregnancies vary in length from 38-42 weeks. It's important to remember that 40 weeks is the median ("middle") of the range and many pregnancies fall on either side of the due date. According to Evidence Based Birth, "only half of pregnant people will go into labor on their own by 40 weeks and 5 days and most people (about 90%) will go into labor on their own by 42 weeks."

"Only half of pregnant people will go into labor on their own by 40 weeks and 5 days."

Instead of thinking of your pregnancy ending in a "due date," you should think of it is as a "due month." I know it's exciting to have that exact date to share with friends and family, but thinking of it as more of a "guestimate" is important for many reasons. Many women experience a high amount of stress when they go past their due date, thinking that there must be something wrong with them, or their baby. Well meaning friends and family send messages, probing for information...."When are you going to have that baby?" "Still pregnant?" Or my personal favorite, "I was sure you would have had that baby by now!" Giving friends and family a due month relieves some of that pressure to have your baby on time!

What happens if I go "overdue?"

Contrary to popular opinion, your due date is not an "expiration date." In fact, only 5% of babies are born on their due dates. (I am one of the few that have very punctual babies. Aria was born the day before her due date. Ellie was born on her due date.) As discussed earlier, normal pregnancies last between 38 and 42 weeks. However, giving birth outside of the norm is viewed very differently depending on if you go "early" or "late." Most care providers are not as concerned with babies born at 37 weeks as they are with babies born past 42 weeks. In fact, many obstetricians will recommend an induction at 41 weeks to make sure that your baby is born before the 42 week "cut-off."

The reason for induction prior to 42 weeks is that research has found that elective induction at 41-42 weeks can lower the risk of stillbirth. The risk of stillbirth increases from 3.5/10,000 (0.003%) at 39 weeks to 10.8/10,000 (0.1%) at 42 weeks. While this number is still very low, it represents a low to moderate risk for most women. 

To induce....or not induce...?

In the case of an "overdue" pregnancy, elective inductions can lower the risk of stillbirth, but they come with their own risks, including uterine hyperstimulation, fetal distress, infection, increased pain in labor, and increased cesarean rates. Inductions also take a long time, especially if it's your first baby. Ultimately, its up to each birthing person to discuss their goals and desires for labor with their provider, and weigh the risks and benefits of induction vs. going into labor naturally. If you're unsure how to talk to your provider, bring your partner or doula to your appointment to facilitate that conversation. You are the "driver" of your birth journey, and this decision is yours to make!

How accurate was your due date? Leave a comment below and share your story!

To learn about due dates and more, sign-up for a Childbirth Class today!

Information in this blog should not be taken as medical advice. Any questions regarding your care during pregnancy, birth, or postpartum should be discussed with your provider.