Today is Valentine’s Day. As such, you can expect to see lots of posts from midwives and doulas–even us–about how we’ll see you in November. How can we be so sure? Well, we can’t. But we just take our best guess.
Your due date, or EDD (estimated due date) or EDC (estimated date of confinement) if you want to get even more old fashioned, is based on an old obstetrical standby: Naegele’s Rule. To determine your EDD using Naegele’s Rule, you subtract three months from the first day of your last period and add one week. So, babies conceived today, are due on November 6th (it’s Leap Year). However, Naegele’s Rule makes lots of assumptions. It assumes that pregnancy lasts 280 days. It assumes that every menstruating person has 28 day cycles. And it assumes that ovulation occurs on day 14, like clockwork. Well, you know what they say when you assume…
In truth, there is a lot of variation in menstruation: longer cycles, shorter cycles; longer ovulatory phases/shorter luteal phases and vice versa. There can be interruptions to regular patterns and surprise ovulations. Breastfeeding can throw another wrench in the curve–some lactating people get pregnant before they’ve even had a period! In essence, anything goes.
And all that being said, Naegele’s rule hits the nail on the head a decent number of times. Still, there are much better ways to date a pregnancy. If you track your cycles with regularity, and take note of signs of ovulation and days you had intercourse, you can increase the accuracy of your dates immensely. But the most accurate clinical method of dating a pregnancy is a transvaginal, first trimester ultrasound. This scan can date a pregnancy within 5 days on either side of the EDD. A 20-week ultrasound (when most people get one) is accurate +/-10 days, and a 3rd trimester ultrasound is only accurate to 3-4 weeks on either side (so a margin of error of 6-8 weeks). You can read more about this here.
And why do we care? Won’t babies come when they’re ready? Well, yes and no. The vast majority of babies will be born between weeks 37-42. But it is essential that labor commences during this window for proceeding with a planned home birth in Colorado–so having inaccurate dates could mean risking someone whose baby is actually full term out of care. In the medical context, inaccurate dates could mean inducing premature babies, or not following post-dates pregnancies close enough, which could be very risky indeed.
So, if you get pregnant tonight, will be at your birth on November 6th? Probably not. But even with a precisely accurate due date, this remains true. Babies do what they want.