Here's How Your Due Date Is Actually Calculated

When I was pregnant, I clung to my due date like a shipwrecked woman clings to a barrel, despite the fact my doctor told me I was highly unlikely to deliver on that day. In fact, she suggested I think of it as more of a due month. Didn't matter. Pregnancy is heavy with uncertainties, and I needed something to hold onto. Though a due date can't predict your baby's exact birthday, it's an important estimation. In fact, your baby's health may depend on it. If you're wondering how to calculate your due date, here's what you need to know.

Doctors calculate your due date by adding 40 weeks to the first day of your last menstrual period (LMP). The trouble is that not all women remember the date of their last period (I never do), and not all women operate on a 28-day cycle. In fact, the Office on Women's Health from the U.S. Department of Health and Human Service reported that in adult women, menstrual cycles range from 21 to 35 days, which leaves lots of room for error. In teenagers, cycles can range from 21 to 45 days.

Romper reached out to Seth Plancher, MD, of Garden City Obstetrics and Gynecology, who confirmed that due dates can be strangely elusive, not to mention confusing. According to Plancher, even some practitioners don't always get it right. He notes,

"You hear a lot about changing due dates, not early on, but somewhere in the middle of the pregnancy. To me, if you’re changing your due date, you didn’t have a good one to start with."

And that's a problem. If your due date is off by just 10 days, and you're scheduled for a planned C-section, you could give birth to a baby that's not ready. On the other end, doctors usually let you deliver one to two weeks late, but if your due date's wrong, you could wind up going two to three weeks late. A post-term pregnancy greatly increases the risk of stillbirth, according to Medscape, and most post-term pregnancies are a direct result from inaccurate dating. So how do you get the most accurate date? For Dr. Plancher, an early ultrasound is crucial — unfortunately, they're not standard.

"A lot of doctors won't do an ultrasound until eight weeks or nine weeks along, and I think exactly the opposite. I like to see people at six weeks," Plancher says. Earlier ultrasounds are most precise, and by week six, your doctor can measure fetal crown-rump length (CRL) — your baby's length from head to butt. As noted in Ultrasoundpaedia, your baby grows 1 millimeter per day from six to nine and a half weeks, making an early CRL a highly accurate way of dating your pregnancy. After week 12, an ultrasound's accuracy as a due date estimator drops significantly.

Plenty of pregnancies come as a surprise, of course, and unfortunately, many women simply don't have access to proper prenatal care, reported Child Health USA. For women coming in at 18 or even 20 weeks, doctors just have to make their best guess based on ultrasound measurements with a wide margin of error.

Don't panic if you got the standard eight-week ultrasound, though. As Plancher tells Romper, that measurement is still probably pretty accurate. Nevertheless, he maintains that earlier is better. To get your most accurate due date, ask your doctor to measure your baby at six weeks if possible. Correlate that measurement with your LMP, and call it a day.