The Truth About the Estimated Due Date

due date marked on calendar

"I am here, ready to have my baby," an expectant mother stated, as a matter of fact, upon arrival at the hospital one morning.

"How far apart are your contractions?" the staff asked, retrieving paperwork to start admitting her.

"I haven't had any contractions," she replied.

"Did your water break?" asked a labor and delivery nurse, overhearing the conversation.


"Are you in pain, are you scheduled to induce, or instructed by your doctor?" the confused nurse searched for what brought this apparently healthy, 9-month-pregnant woman to the E.R. on a normal day.

"No," the woman countered. "But, today is my due date. So, I'm here to have my baby who is due today."

Astonishing enough, this interaction is far from the only story like it. Many women seem to think of the "due date" as more of a deadline, rather than an estimation. And it is not uncommon for expectant mothers and even doctors, to panic when a woman goes past her 40 weeks, even by just a couple of days.

Understanding Due Dates

Is a baby like a library book that's overdue, or needs to be checked out on time? Of course not. Each baby and each pregnancy is different. So, why the assembly-line birth formulas? What does the estimated due date really mean? Is it safe to go past 40 weeks? Or should you schedule invasive measures at, say, 40 weeks, day 2?

pregnant mom holding blocks that say week 40

Only 5% of mothers give birth on their expected due date. Five percent. That means that 95% of the time, babies come on a time table that is anything other than what moms or doctors plan. Approximately 80 percent of women deliver sometime between 37 and 42 weeks.

In Janelle Durham's research article on Calculating Due Dates and the Impact of Mistaken Estimates of Gestational Age, she explores the mental stress of society's pressure of the EDD. "The predicted due date has an impact on the psychology of pregnant women and their partners," writes Durham. "Although their physician or childbirth educator may have told them that the 'due date' is only an estimate, many modern American women take it as a scheduled date around which to make their plans. Some parts of popular culture, like baby betting pools, remind us that the date is difficult to guess; however, other messages imply that we can expect a certain date. The Girlfriend's Guide to Pregnancy even has a countdown calendar to the due date… there is a token message there saying something like 'baby's not here yet? Well, take care of yourself and be patient.' But clearly, the message is to expect the baby then, despite the fact that only 4-5% of babies are born on their exact due date."

So, if EDDs Aren't a Crystal Ball, What Good Are They?

The most common way to calculate a due date is to set it at 40 weeks (280 days) past the woman's last menstrual period (LMP). This is known as Naegele's Rule, after a German doctor who published the method based on the calculations of a 1744 botanist, around the year 1812. According to "The Lie of the EDD: Why Your Due Date Isn't When You Think" by Misha Safranski, the theory calculates human gestation lasting approximately 10 lunar months. "There is one glaring flaw in Naegele's rule," says Safranski. "Strictly speaking, a lunar (or synodic - from new moon to new moon) month is actually 29.53 days, which makes 10 lunar months roughly 295 days, a full 15 days longer than the 280 days gestation we've been lead to believe is average."

The standard EDD method assumes that the woman's period arrives like clockwork every 28 days. If your cycles are longer, you're likely to deliver later than your due date; if they're shorter, expect to deliver earlier.

One of the reasons the standard is still practiced today is because, give or take a few days, the method is still the median average. A 1999 study of over 17,000 spontaneous deliveries stated that due dates are more accurate if we add 282 days to LMP instead of 280 ( Evaluation of ultrasound-estimated date of delivery in 17,450 spontaneous singleton births: do we need to modify Naegele's rule? Nguyen TH, Larsen T, Engholm G, Møller H .) However, as long as the baby and mother are not in distress, there is absolutely no reason the mother can't be allowed to naturally progress to 42 weeks before discussing any alternatives.

pregnant mom writing in day planner

Aside from the LMP, doctors can use ultrasounds for calculating your due date. A first-trimester sonogram can be considered more trustworthy for estimating when a baby is expected. Your doctor will measure the length of the fetus and the size of gestation. If these numbers don't match up with what your LMP would predict, your due date can be adjusted accordingly. The best time to measure for the most accurate EDD, according to New Birth Company for birth centers and midwives, is during the first 12 weeks of the pregnancy. The ultrasound machine calculates your baby and compares it to the "average size" of a baby at a certain growth and "estimates" a due date within one week. The accuracy of this method drops the further along you are. An ultrasound done after 12 weeks has an error margin of 2 full weeks. After 28 weeks of pregnancy, the margin of error for an EDD from an ultrasound jumps up to 3 weeks.

No matter what method, your due date is simply estimated, not guaranteed. No womb has an exact predictable schedule of when labor will start. The important thing is that your baby is healthy and protected, arriving when your body and baby are ready and safest.