Why is 40 weeks the magical number for every pregnancy?
Forty weeks has become the standard gestational (gestation means time baby is in utero) length for humans. Why? German obstetrician Franz Karl Naegele (in the 1800’s) came up with a method to determine a woman’s due date based on her last menstrual period (LMP).
Essentially the dealio is this:
- Take your last menstrual period.
- Subtract three months
- Add 7 days.
- Voila, you’ve got “the date”
But what if you don’t have a 28-day cycle? What if you ovulate on different days of the month? Rut row! Naegele, in all his German-ness, didn’t factor these in. “Houston, we have a problem”… with the Naegele Method, because bottom line, it assumes every woman has their period & ovulates with German engineering precision.
The other assumption made by our German friend Dr. Franz, is that pregnancy lasts 280 days. How we got to this number is interesting. 40 weeks is the midpoint of the “normal range” of a “normal pregnancy”- there have been studies across the world on this.
So if you were to look at a bell curve of women and when they give birth, it would look something like the illustration below.
40 weeks would be at the top of the curve with an equal number of women giving birth before & after 40 weeks (an easy way of saying a normal standard deviation). A pregnancy is considered “full term” anytime 2 weeks before & 2 weeks after that 40 week midpoint (38-42 weeks).
In other words, 40 weeks is not a magic number.
Since each woman & baby are unique, it makes sense the development and ‘readiness’ of each baby will differ & will vary by as much as 2 weeks on either side. We should expect a normal 2 week variation of when births occur.
Fruit Trees & Babies…Huh? Why does this matter to me anyway?
Think about the babies you know (heck- think about any kid or even adult you know). They reach milestones at different ages. Like rolling over, or scooting, or crawling. Eventually they start pulling themselves up on tables and couches…inevitably taking their first step when they’re ready. If you did an informal survey of your friends’ kids, you’ll see not every child hits these developmental markers at the same time or in the same way. Each baby is different. For lots of reasons.
You don’t see parents buying their baby “baby walkers” to expedite their learning to walk at the 11 month mark. Yet this is precisely what we’re doing with babies in utero when we induce for no medical reason (unlike inducing for medical reasons like preeclampsia- means mom has high blood pressure, etc.). Inducing has risks to baby.
41% of respondents in the Childbirth Connection “Listening to Mothers Survey” (almost 1,600 women) said their provider tried to induce labor with drugs, and 84% indicated that the induction worked. 25% of the women taking the survey said the induction was due to “being overdue”, and 19% of the mother’s just ‘wanted to be done’ with pregnancy. The mothers who cited “overdue” as reason for induction, on average, gave birth at 41 weeks, while mothers who cited other reasons gave birth (on average) at 39 weeks.
A growing number of doctors & experts will tell you that inducing birth before 39 weeks is dangerous. It’s dangerous for babies & their development (Evidence here). During the last 2 weeks baby is in the womb, there’s a big time surge of brain development, lung development and maturity of organs, which is critical to baby’s health at birth, and as they grow. Evidence here.
Something to think about…If due dates aren’t an exact science (research shows they’re not), if we can’t be exactly sure when we conceived, if each baby develops uniquely at different rates in the womb & if science tells us there is big time critical organ development (brain, lung & others) at the tail end of baby’s time in the womb…then why induce (assuming no medical need) before baby signals they’re ready (babies actually emit a hormone triggering the mom’s body into spontaneous labor)? Especially with Induction introduces it’s own set of risks? Is it worth inducing for mom’s convenience & wanting to be done?
These are only questions you can answer. Maybe it is. It’s your decision. But one thing’s for certain, this important information for you to consider in your decision is massively under-communicated. Get all the info so you make the best informed decision for you!
Ultrasound as a tool.
Some providers we’ve spoken to are using ultrasound during the first trimester to confirm how far along baby is. They take the date of the mother’s last period and compare it to the ultrasound measurements taken of baby. The accuracy of ultrasound is highest early in the pregnancy (before 18 weeks).
Ultrasounds in late pregnancy are not as accurate, they can be off by as much as 2 pounds either way. Having early confirmation of how far along you are, based on accurate info, can be super helpful in making informed decisions later in your pregnancy. Especially if discussions need to happen regarding induction, baby being too big, too small, etc. You’ll have a good accurate ‘baseline’ from which you can make better decisions, reduce the chances of unnecessary interventions & identify potential complications (supporting research here from the American Journal of Obstetrics & Gynecology). And check out our Get Smart Article on Induction here.
These are things no one talks about until after they’ve learned the hard way. That’s just silly. Let’s all get it all out and learn from each other BEFORE we have to make the tough decisions. If you’re in the dark about when you conceived, ask your provider about getting a first trimester ultrasound. If it’s not something they typically do, then point them to the research above and chat with them about why it’s important to you. Afterall, it’s your pregnancy & baby.
In fact, this exact point was one of Dr. Lucky Jain’s (neonatologist from Emory University- he specializes in high risk births) main takeaways…get accurate info early because it can help you big time later.
There is no A+B=C equation to figure out exactly when baby is arriving. Labor begins when the baby emits hormones signaling their bodies are ready for the outside world. It’s difficult not to see the due date as “the date written in stone that my baby will be arriving & I’ll be planning my life & scheduling around it” date.
People, as great as their intentions are, want to know THE DATE you’re due and will pester you endlessly about baby’s arrival when they know you’re getting close. With the advent of social media, this can be tough (mentally) on mama. It’s hard enough being uncomfortable but now your uterus is a “watched pot” by your hundreds of friends on Facebook, and we all know what happens when you stand there looking at your pot— it takes FOREVERRRRR to boil.
Here’s a suggestion: Maybe instead of focusing on a “due date”, we focus on a “due month” or “due season” and let baby come when he’s ready. When people ask you when your due date is, you could say “beginning of (birth month)”. It will help you mentally and will communicate in a nice way that just because your estimated due date (EDD) is June 6th, it does not at all mean baby will arrive that day. If you want, you can always bust out the bell curve illustration & bring up our homeboy Dr. Franz Naegele . Then when people start getting glassy-eyed…drop the ‘standard deviation on Naegele’s curve’ lingo on ‘em & they’ll probably run away with Statistics Class flashbacks never asking you again:)
Check out our Interview with Dr. Lucky Jain from the Emory University School of Medicine on induction. Not only is he a big time leader in medicine, he’s also the man behind a lot of the research.
“Impatience never commanded success.” Edwin H. Chapin