What’s In A Due Date? {Part I}

It’s the very first question you hear when someone discovers you are expecting.

“When are you due?”

My first pregnancy, I was obsessed with my due date. I knew, to the day, how far along I was (23 weeks, 5 days!) and eagerly counted down the months, weeks, and days with both excitement and anxiousness. This mindset also dominated the many pregnancy forums I had subscribed to, with women constantly comparing dates, symptoms, weight gain, and bellies; even the apps had a day-to-day update of how your baby was developing, how much he or she should weigh, and what movements you should be expecting. When someone asked me when I was due, I proudly exclaimed “November 12!”

This time around, however, is a little different. If you are pregnant, try giving just a month answer when someone asks when you are due. It’s amazing how replying with a simple “May” instead of “May 15th” makes people react a little differently (usually with the follow-up question, “When in May?”). If you are super brave, I dare you to reply with a season (“this spring”) – that will really drive people crazy!

I totally get it. Pregnancy is an exciting thing, and we love to have something to look forward to. With the increased use of technology, we have made assigning a newborn’s birthdate a completely normal and expected part of pregnancy. In itself, not necessarily a negative thing – until that date approaches, and baby has yet to make an appearance. Then what?


Before we get into that, I want to talk a bit about how due dates are calculated.

1. Last Menstruation Cycle

At your first prenatal exam, your nurse/doctor will ask you the date of your last period. The theory behind this is that every woman has a menstrual cycle of 28 days, and that every woman ovulates halfway through that cycle, at day 14. (Ovulation is when the egg is released from the ovaries – which is when conception occurs.)

Raise your hand if you have a period every four weeks (naturally, not while using birth control).

This is the first generalization we have made about women: that our bodies are machines, they should all run the exact same way, and if they do not, they are somehow flawed. The truth is, women can ovulate more or less often, changing on a monthly basis, and still be completely normal or healthy. My own cycles tend to be a bit longer, between 30-40 days on average, but that can change depending on many factors. When I discovered I was expecting my daughter, I had been off birth control for about a year. Because I had no idea what my body would do naturally, without all those hormones, I had been charting my cycles, symptoms, etc. Therefore, I pretty much knew exactly when she was conceived. However, because it was outside that “four week” theory, my doctor assumed I was several weeks farther along than I actually was. What was really frustrating was that I KNEW my body, yet whenever I told a nurse my conception date (these conversations happened at least several times), they would just kind of nod and smile in a placating way, like they – or that little circle date guesser- knew better.

Same with my second. I hadn’t been charting like I did with my first, but I had a pretty general idea of when conception occurred. This time, however, we opted to go with a midwife, and while one of her first questions was about my missed period, when I explained to her how my cycles usually occur and how far along I thought I was, she actually listened to me and even – to my surprise – set my due date based on my knowledge. That was very empowering, to be trusted about my own body.


2. Ultrasound

Every doctor/midwife and birthing practice is different, but it seems like, in most cases, the next step is performing an ultrasound. With my first, my doctor was actually a bit more conservative and didn’t seem eager to perform a first-trimester ultrasound. However, at my first “real” appointment (at my nurse-designated 12-weeks), he could tell by palpation that I didn’t feel quite as far along as I should have. Therefore, he did order an ultrasound, and guess what they found out? Yep – I was (to the date, no less!) exactly as far along as I had figured from my own charting. (And, yes, I did gloat a little – but only to my husband.)

In other practices, you may receive an ultrasound right away to confirm pregnancy (if this is before 8-10 weeks, be prepared to not see a whole lot; sometimes at such an early gestation, you may or may not even be able to hear a heartbeat, even though it is there!). In yet others, like with my midwife, you may not receive one at all – and that is perfectly okay, too. I can’t count the number of times where I read on a forum or talked to a friend whose doctor, for whatever reason, did not opt to perform an early ultrasound- usually to the dismay of the pregnant woman. We have been so conditioned that an ultrasound is “necessary”, when we do not receive one, we start to worry. “What if I’m not pregnant?” “What if something is wrong?” It is at this point where faith is so important. Ultrasounds are a relatively new procedure – even a generation before us did not utilize them to the extent that we do now, and even now more and more women are opting not to have them done at all due to controversy about how safe they actually are (more on this in a coming post). In this case, just relax, take care of yourself the best you can, and trust that baby is growing as he or she should.

If, however, you do receive an early ultrasound, chances are their gestation calculation will be a tad more accurate than just relying on menstruation alone.

3. 40 Weeks

While your period and/or an ultrasound can, for the most part, decipher when you conceived, the idea that a normal, healthy pregnancy is 40 weeks is what firmly anchors your due date in the medical community. Unfortunately, this is yet another misconception about women’s bodies. Can I just reiterate? We are not machines. Each women – and each baby – is special and unique. While it’s nice to be able to have a general idea of how long a baby needs to “cook”, it’s not a time frame set in stone, and – no matter what you are told – you are not “overdue” or “expired” if your baby happens to want to wait beyond those 40 weeks.

So how did they come up with 40 weeks, anyway?

The 40-week theory, known as “Naegele’s Rule”, was named for an 1850s German doctor who, through his own observations, calculated that pregnancy lasted for 10 lunar months, which would be 280 days from the first day of the last menstrual cycle or 266 days from the assumed day of ovulation (based on the assumption that all women have 28 day cycles). In 1990, researchers found that, “uncomplicated, spontaneous-labor pregnancy in private-care white mothers is longer than Naegele’s rule predicts”; on average, pregnancy is more likely to last for 274 days from ovulation – a difference of a little over a week! Interestingly, gestation length has also been seen to vary based on whether the mother has given birth before, racial groups, age of the parents, and even how long their own gestations were.

“The concept of a ‘due’ date is something that is very abstract in many cultures. If, like in Uganda, you count a pregnancy by the moons, you end up having a ‘due month’ and baby comes when she is perfectly ripe without causing mama lots of anxiety.” – Mother Health International

I have been looking for studies that have collected data on when babies are born in comparison to their gestation.

For example, this study in Sweden looked at singleton (one baby) births from 1976-1980 and found the average gestation age at birth was 282 days, with 10% of these births occuring ‘post term’, or after 294 days.

Another study in Minnesota compared births from 1994 and 2004 and found that “Overall, there has been a slight shift in the gestational age at which infants are born, with more infants being born at earlier ages in 2004 than in 1994. In 1994, the greatest number of births (24.4%) occurred at 40 weeks gestational age, while in 2004 the greatest number (26.0%) occurred a week earlier at 39 weeks gestation. The biggest changes have occurred between 38 and 41 weeks. In 2004, 43.4% of babies were born between 38 and 39 weeks compared to 37.8% in 1994, a 5.6 percentage point increase, while the percent of babies born in the 40th and 41st weeks have decreased from 38.5% in 1994 to 34.3% 2004.” Ironically, the number of babies born by induction or cesarean also increased 11.2% in those short ten years, which would account for why babies are being born earlier.

This article from the BBC quoted data from the Perinatal Institute that “an estimated date of delivery is rarely accurate – in fact, a baby is born on its predicted due date just 4% of the time” and that “the baby is likely to come any time between 37 weeks (259 days) and 42 weeks (294 days)” with “women … whose pregnancies are straightforward and low risk, 60% of babies are born within a week either side of the estimated date of delivery” and more than 90% are born two weeks either side of the predicted date.

The concept of a 'due' date is something that is very abstract in many cultures. If, like in Uganda, you count a pregnancy by the moons, you end up having a 'due month' and baby comes when she is perfectly ripe without causing mama lots of anxiety. The very talented artist who created this beautiful illustration is Itaiana Battoni - Desenho Artístico.

To pick the fruit before it’s ripe, or to wait for it to come when it’s ready? Art by Itaiana Battoni – Desenho Artístico.

So, with normal and healthy babies being born within an average window of a month, why are more and more mothers opting for inductions?

Continue on to read “What’s In A Due Date? {Part II}”

Leave a Reply