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These facts about your baby’s due date should help you relax



Anna O'Neil - published on 07/06/19

Due dates can cause a lot of anxiety, but even science tells us that it's not an exact science.

I’ve had four years of practice, but to this day, I still forget my son’s actual birth date because I spent almost every day of that pregnancy telling people his due date till it wore a groove in my brain. May 15, May 15, May 15. I gave birth to him before that, and now I look pretty silly when his pediatrician wants to know his birthday, and I have to think about it.

Memory problems aside (I blame the sleep deprivation), it’s actually pretty rare for babies to come when they’re supposed to. Only 5% of babies are actually born on their due dates, even when the date is determined by ultrasound. Only 35% of mothers go into labor during the week of their due date. As for how the date is even calculated, it’s not as exact a science as we’d like it to be. 

The common practice of predicting the due date is by counting 40 weeks, or 280 days, forward from the date of your last menstrual period. That’s only accurate when your cycles are 28 days long, and when you always ovulate on day 14. Unless a woman has been tracking her fertility carefully, and knows the date of ovulation, there’s no guarantee that she ovulates right in the middle. Even if she did, there’s variation in the time it takes for a fertilized egg to implant in the uterine wall. There’s a very broad range of normal when it comes to these things. 

Doctors will also try to pinpoint the due date via anatomy ultrasound, which is most accurate in the first trimester, especially between weeks 8 and 12. That method is more accurate than looking at the calendar, but depending on the ultrasound technician’s skill and individual variables, the due date can still be about five days off. The later you get your scan, the less accurate the measurements become. 

It’s important to have an estimated due date; it helps doctors lower your risk of preterm labor, birth complications, and interventions. But it’s also important to remember that the date is an estimate, not a guarantee.

Factors like your own health, your genetics, your height, the structure of your uterus, and the baby’s own weight can all influence when the baby makes his or her appearance. In fact, a 2013 study of 125 pregnancies found a range of 37 days from ovulation to birth. It concluded that “Human gestational length varies considerably even when measured exactly (from ovulation).”

I know why I held onto my due date so hard with my first child. It was supposed to be the one predictable thing in a pregnancy filled with variables. It felt comforting. But I was lucky. If I had been a few days “late” instead of early, I would have felt cheated — the baby was supposed to be out by now! Not fair!

With my second baby, I knew to answer people’s questions more vaguely: “She’s due around Valentine’s day.” It kept my own expectations in check, and it was much more realistic than pinning all my hopes on one magical day.

Keep your due date in mind, absolutely, but don’t panic if the baby ends up surprising you. Motherhood is going to be surprising no matter what!

Read more:
The five wonders of pregnancy


Read more:
Why you should take those “unreasonable” pregnancy mood swings seriously

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