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The Right Due Date

February 18, 2019

Disclaimer: I’m not a doctor. This is not medical advice, just my interpretation of the facts. Read the cited links and think for yourself before trusting a stranger on the internet.

Short Version

When you find out you’re pregnant, tell your doctors that you don’t remember when your last period was. Then schedule an ultrasound between 8 and 9 weeks after your last period to establish your due date. You’ll get a more accurate date and you’ll be significantly less likely to be artificially induced because your pregnancy is considered late.

Long Version

If you want to minimize the odds of having your labor artificially induced, getting the right due date is very important. The risk of complications increases the longer you go past your date. At 41 weeks (that’s one week late), your doctor or midwife may gently recommend that you consider induction. By the time you cross 42 weeks, and roughly 8% of women do, the anxiety level will be a lot higher and the recommendation a lot stronger. At this late stage, a due date that’s off just a few days in either direction can change the medical interpretation a great deal.

There are two common ways to establish the due date: counting forward from the first day of the mother’s last menstrual period (LMP) and getting an ultrasound. According to the latest guidelines (a) from the American College of Obstetricians and Gynecologists (ACOG), a first trimester ultrasound is the gold standard, and the earlier in the trimester, the more accurate it is. However, ultrasounds before eight weeks run the risk of a fetus that’s too small to measure. So the very best time to do your ultrasound is between 8 and 9 weeks after the first day of your last menstrual period. If you can’t get it scheduled for that exact window, do it as soon afterwards as you can. Anytime up to 14 weeks is fine, but sooner is better.

Here’s where it gets weird: the ACOG also recommends that as soon as a due date is established (whether by LMP or ultrasound), your doctor should enter it into your medical record and should not change it except in “rare circumstances”. Specifically, it should only be updated when the ultrasound disagrees with the LMP date by a significant margin—typically 7 or more days, but it varies based on when the ultrasound was done. So you can end up in a situation where your LMP date is off by almost a week from your ultrasound date, yet the less accurate date is used.

That’s exactly what happened to my wife and me when our daughter was born. Our LMP date said we were at 42 weeks, but according to the ultrasound we were at 41 weeks, 2 days. We came to the hospital for a routine test (you get them every other day when you’re late) and everyone was on high alert and amazed that we had not been induced yet. There was a lot of pressure to speed the labor along artificially, and no one cared that the more accurate dating gave us plenty of time to wait. We felt that our caregivers were not on our side, at a time when we were most vulnerable. Luckily, my wife went into labor shortly thereafter and there were no complications.

To avoid these problems, make sure your ultrasound date becomes the official date. Simply asking your doctor won’t work. They will most likely refuse to deviate from the ACOG guidelines because it opens them up to a potential lawsuit down the road (and lawsuits are already super common in obstetrics).

Instead, keep your LMP date to yourself. When your doctor asks you, say that you weren’t paying attention and you don’t remember (it happens more often than you think). Then schedule your ultrasound at the right time, and you’ll get the most accurate due date you can.

If you know anyone who may be thinking of getting pregnant, send them this post. By the time they tell you they are pregnant, it may be too late. They’ve probably already confirmed their pregnancy with a doctor and have a due date on file.