How Does A Due Date Calculator Work?

Pregnancy due dates are notoriously unreliable. Here's how to calculate your due date, and what it means when that is way off from the gestational age.

Babies are born after nine months—sort of. Pregnancy due dates are notoriously unreliable. For many people, ball-parking infant ETAs involves entering menstrual, height and weight, and coffee drinking data into a due date calculator in service of getting a number that is all but guaranteed to be wrong. The next step? Getting an ultrasound that misassigns a “gestational age.” The question expecting couples find themselves struggling with is this: What does any of it mean? Essentially, it means that there is a flawed way of calculating a birth date to within a margin of error. The better you understand the margin, the better you understand what to expect (and how to create a birth plan).

We’re here to help. Here’s how to calculate your due date like a scientist:

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The Basics: Naegele’s Rule

Online due date calculators generally rely on the simple, if somewhat inaccurate, Naegele’s Rule. Franz Naegele, a German obstetrician, came up with his eponymous rule in the early 1800’s. He predicted childbirth exactly 280 days after the last menstrual period before getting pregnant.

Naegele’s thinking was simple enough. The average menstrual cycle is 28 days long, with ovulation occurring at day 14. With a little elbow grease, you can make Naegele’s ballpark estimate even more accurate by adding one day to the due date for every day your cycle is longer than 28 days, and subtracting one day from the due day for every day that it’s shorter.

Only about 4 percent of babies are born on their due dates as determined by Naegele’s Rule. For a more accurate guess, add two months to the due date in either direction. This will give you a “due month”, and 80 percent of babies will arrive within that window. Or, put another way, you can set a reasonably reliable baby deadline by calculating 294 days from your last period.

Alternatives: The Mittendorf-Williams Rule

Nearly two centuries after Naegael took obstetrics by storm, physicians Robert Mittendorf and Michelle Williams developed a computer model for estimating due dates that considered 16 factors, including maternal age, race, weight, coffee intake, and pregnancy history. The Mittendorf-Williams predicts that the average pregnancy lasts 288 after the last menstrual period for Caucasian first-time mothers, and 283 days for Caucasian non-first-time moms. The rule is more unwieldy, but studies suggest it’s at least twice as accurate as Naegele’s rule.

What Can’t I Just Calculate From The Date We Conceived?

Well, you can. But that would mean knowing exactly when you conceived. It’s trickier business than you might think. Even if you do know exactly what day you had sex, sperm can live in the woman’s body for up to five days, and the egg can live for up to 24 hours after being released. As a rule of thumb, some experts suggest adding two days to the date of intercourse and counting from then. But the Mittendorf-Williams rule is probably still most accurate.  

What is This Gestational Age Business?

Depends on which gestational age you’re talking about. Conceptional age is the age of the baby from when he or she was conceived. Gestational age is the age of the baby from the last menstrual period, which should fall about two weeks earlier than that.

On an ultrasound, however, gestational age means something entirely different. Here, the ultrasound technician is guessing the baby’s age based on its measurements as opposed to calculating from conception or last period. It is normal for gestational age on an ultrasound to not perfectly match the calculated age of a fetus—but large discrepancies can be signs of developmental problems (or merely a sign that the due date was grossly miscalculated).

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