Medical Examiner

Stop Measuring Pregnancy in Months!

One doctor’s plea against this inaccurate—and misleading—way of talking about human gestation.

pregnancy is 9 months.
How many weeks along?


There are so few things I’m really, really adamant about as a women’s health provider. I mean, aside from the basic necessities (evidence-based medicine, good care, patient autonomy), I don’t hold by much ceremony in my office. You want to bring your kids to visits? Totally OK; we’ll provide crayons. You need to have extremely loud heavy metal music on while you deliver? Hey, it’s your labor.

But I have found my tiny back-breaking straw, the nagging issue that will send me around the bend. It is my patients’ need to talk about pregnancy in terms of months. That is, “Hey, Doc. How many months pregnant am I?”

The reason this drives me crazy is simple. It is because I can’t tell them. Because months are a giant misleading disaster when it comes to talking about pregnancy.

Here’s how it works. When we start caring for a pregnancy, one of the first and most important things we have to do is figure out how long it has been going on. The nomenclature we use to do this is an estimated due date. An estimated due date is calculated as 40 weeks out from your last menstrual period, and 38 weeks from what we somewhat theoretically ascertain as your ovulation and conception. We can utilize your menstrual history or an ultrasound to figure out these numbers.

Calling it an estimated due date at all is misleading terminology. It isn’t, after all, the date I’m expecting the baby will arrive—I’m not generally in charge of that timing, nor am I in the business of predicting the future. Your EDD is simply the date this pregnancy will be at 40 weeks’ gestation, no more and no less. This is ancient pregnancy wisdom as well as historical convention, but modern observations have confirmed that human gestation, on average, lasts approximately 280 days after the last menstrual period, so that part makes sense.

But if the EDD doesn’t explain what day the baby will actually be delivered, what is it good for? Well, still quite a lot, actually. The EDD is how we describe how old the pregnancy is. It allows us to synchronize all of our pregnancy watches with the necessary events (for example, you need a Group B strep test at 36 weeks). It lets us meaningfully compare your pregnancy to other pregnancies its age. (When we measure your baby, we want to make sure we’re comparing its size to other fetuses that are the same age). In both of these instances, weeks are preferable because they are more precise.

The unit of “months” is so largely inaccurate that it renders most conversations useless for the purposes of medical care. For example, let’s say you had your first baby early, a scary case of preterm birth. You tell me: “She was born at seven months.” Well, how long is seven months? Is it 32 weeks (40 weeks minus two months)? Or is it 7 x 4(ish) = 28 weeks? As anyone who’s had a baby in the neonatal intensive care unit can tell you, the difference between a 28-week baby and a 32-week one is actually a million years and several metric tons of heartbreak. I really need to know what happened when in order to take care of you properly; and utilizing months makes it particularly hard to parse.

Another terrible side effect of describing a pregnancy in months—and nine of those, to be exact—is that it makes all pregnancy feel longer. If your last menstrual period began on New Year’s Day, your EDD would be Oct. 7. But hey, did you notice something? Jan. 1 to Oct. 7, that’s not nine months. That’s, like, nine months and a bit. Because—and here’s the part with the misleading disaster—months are a terrible unit in which to measure time. Sure, a month is basically four weeks, but they’re (mostly) all actually a little bit longer, and by different amounts. There’s technically 4.3 weeks in each month, in a very nonexact sort of way, and so a 40-week pregnancy is more like 9.3 months.

That .3 may feel minor and sort of like a rounding error. But it causes me, as a pregnancy doctor, problems. Pregnancy is never over when women think it’s going to be. And this ends up being particularly cruel, because the last month is for many the most uncomfortable—so that 0.3 months tacked there at the end is for many, unexpected, and even more unbearable. It’s like telling someone who reaches the end of a marathon that Actually, it’s gonna be a 27-mile race this time, hope that’s cool.

Thus, in the interests of both morale and more accurate medical care, I have tried to get my patients to divest themselves of months. Weeks! I say. So much more helpful to both of us. So much more nuance! And so fun, right? Because I think we should have a little mini-party whenever you turn a new week—that’s a party every seven days. Right? Right?

So far, no takers. Months it is, all the way around. I’m not sure why. Is it because, as women, we’ve been counting time by months since the first day we got our first period? The norm outside of the doctor’s office is certainly to take about your pregnancy in terms of months. Months are pervasive through culture and religion; Christmas, after all, is nine months after the Feast of the Annunciation, and not 40 weeks.

For whatever reason, the end result is powerful: Everyone wants months; everyone’s grandmother and boyfriend and co-worker want months. Months have a strong, powerful, collective grasp on the social experience of pregnancy, and I know when I’m beat. I’ll get used to it, probably in the next few fortnights or so.