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Your Due Date Is Wrong.

Both of my daughters arrived “late” according to the dates in my charts. Both girls seemed to mock the very concept of a “Due Date”. The little one, especially—she showed up a mere 30 minutes before my scheduled induction, 11 days after the day she was clinically expected to arrive.

I am not alone. The chance of delivering exactly on your magically assigned date is rare — only 5% of women will deliver on their Estimated Due Date.

So there’s a pretty good chance that your due date is wrong too.

But why are we so bad at accurately predicting due dates?

Due date prediction is an imperfect science

Assigning a due date takes several factors into account: last menstrual period as a proxy for the assumed day of ovulation, size of the baby at various stages (assessed via ultrasound).

A study compared these traditional methods in the cases of nearly 20,000 births. The researchers found that precise prediction is basically impossible. Regardless of which method they used to determine estimated Date of Birth, the actual Date of Birth showed considerable variation between babies (up to 2 weeks before, and 2 weeks after). Even with the earliest ultrasound, between 11–14 weeks when the fetus is lime-sized, the size of said lime appears to vary just enough to screw up any degree of predictability.


“Expectant mothers should be informed that there is only a 35% chance that they will actually go into labor during the week of their Estimated Date of Birth” – Khambalia, et al. 2001

Your Estimated Due Date (give or take a few weeks)

Forget those fancy tools of due date prediction, a 2015 research study proved out what we’ve always known – babies don’t play by our rules.

In this study, researchers found that gestation lengths for normal pregnancies can vary up to 5 weeks. FIVE WEEKS! The researchers in this study knew exactly which day the little egg popped out of the ovary and started its journey to becoming a human. No guessing on last period or assuming ovulation date. This method is precision timing at its best.


Gestation lengths for normal pregnancies can vary up to 5 weeks


Five weeks is obviously a huge window. This study suggests that human gestation may be a bit more complicated than previously expected. Even after the sperm race concludes and genetic material combines, the uterus-bound ball of cells does not follow a consistent trajectory. It can take the fast, direct route or the slow, scenic route before it nuzzles its way into the uterine lining and sets up shop. Even crazier, the environment that mom provides for the little ball of cells can also influence timing. All these little influences along the way then lead to a shifted developmental timeline that throws off our current methods of predicting due dates.

Essentially, each tiny human can be fast out of the gate or slow. Fast embryos are born sooner.


“The trajectory for the timing of delivery may be set in early pregnancy” — Jukic, et al. 2013

Genetic factors for your real Due Date

Comparisons of mom and dad’s birth record vs. their babies birth date also suggest that genetics play an important role in due date prediction. Simply put, faster-growing babies hit escape trigger earlier than slower growing babies.

But is the trigger baby size? or size of the remaining space in the uterus?

Seems like it’s the latter: babies squished into tight spaces may pull the trigger earlier! Short moms (>5’3”) deliver their babies almost five days earlier than their tall mom counterparts (>5’6”). But it’s not all genetic effects on baby size – tall or short dads had zero influence on when baby arrived. And babies of tall moms tend to come post due date, suggesting that the extra space may stall baby escape plans. Biologically speaking, uterine stretch may have something to do with when and how the body starts the labor process.


Essentially, each tiny human can be fast out of the gate or slow.

Fast embryos are born sooner.

Do Due Dates even matter?

We, as mothers and fathers, and anyone who has ever hounded a pregnant woman with the “when are you due?” question expect and crave predictability. We want a due “date”. But beyond satisfying the curious neighbor or aunt, there are serious implications for placing too much emphasis on a specific date.

If we don’t really know what the actual gestation length should be for each specific baby, how do we know who is early, who is late, and who is right on time?

Due dates serve an important clinical purpose—they provide information on when to intervene, speed things along, and get that baby out.

This may happen too early, before baby is ready. That is bad.

Or, it may happen too late. That is really bad.

One study found that baby girls have later estimated due dates than they probably should. As a result, girls have a higher risk of going post-term with serious consequences. Emphasizing the need to balance consequences of unnecessary early intervention with the possibility of intervening too late, the researchers on that study suggest a simple solution. They advise keeping a close eye on pregnancies with a wider window consideration of “post-date” and allow mom to have a strong voice in the decision about when to intervene. 

They suggest putting all the cards on the table:

“Let women make an informed decision about which management they prefer… fully inform mothers about the uncertainties of pregnancy dating.” — Skalkido, et al. 2010


It doesn’t clear up the uncertainty but including women in the decision making process with full transparency about the BS of due date calculation, seems like a step in the right direction.

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Molly Dickens, PhD

About Molly

Molly has her PhD in Physiology and spent over a decade as an academic research scientist slightly obsessed with the colliding worlds of brain science, hormones, stress and the reproductive system. Nowadays she heads up Content and Community at Bloomlife and edits Preg U. Science is still her jam and she can't help but continue to dive into the research world to find interesting bits about pregnancy and parenting.


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