When Is My Due Date? 5 Ways To Determine When You're Having This Baby

The most common question I was asked during my pregnancy wasn't, "Can I hold the door for you?" Or, "Would you like a nap?" All anyone wanted to know was when I was due. "When is my due date?" was the first question to cross my mind too, but I more or less put it out of my head after my doctor explained that due dates aren't all that informative — for patients, anyway. "No one gives birth on their due date," she told me, as I excitedly marked November 14th in the calendar on my phone. "It's really more like a due month."

Romper reached out to Sara Twogood, MD, OB-GYN and contributor to The Bump, for information about how to calculate your due date. She kindly explains that women can get a due date at home by calculating from their last period or using an online due date calculator, but she was careful to reinforce that "the most accurate dating is with a first trimester ultrasound." Because your due date will follow you throughout your pregnancy, trust me — you want a good one.

While due dates are an imprecise reference point for pregnant women and their families, they're crucial for providers. The primary job of obstetricians managing a normal pregnancy is to watch, wait, and measure. They're interested in how your weight, blood sugar, fundal height, and the size of your fetus stack up against the average woman's pregnancy, because a major deviation can give them a head's up if something goes wrong. An accurate due date is the starting point for a host of critical calculations that really matter to the health of mom and baby.

1With A Little Math

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One way to calculate your due date at home is with a little math. Just add 280 days (40 weeks) to the date of your last menstrual period (LMP). You can do this with a desk planner or calendar. (Keep in mind that this method assumes a 28-day cycle. If you know that your cycle is short or long, it's best to head to the doctor for an ultrasound.)

What you'll notice, calculating this way, is that for the first two weeks of your pregnancy, you're not really pregnant. I mean, you didn't conceive on your period, right? You conceived somewhere in the weeks that followed. That's OK. Calculating your due date isn't like RSVPing for a wedding. Only 5 percent of births take place on the mother's due date, according to YourDueDate.com. Frustrating? Maybe. Exciting? Absolutely.

2With A Due Calculator

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Personally, I don't even have a physical calendar, because smartphones. If math's not your thing, you probably won't be surprised to find that when it comes to due dates, there's an app for that.

"A due date calculator can be found online — I use the one from the American College of Obstetricians and Gynecologists (ACOG) or Perinatology.com, and I love that The Bump calculator tells you the size of your baby compared to fruits and veggies," explains Twogood.

Again, these due date calculators are designed for women with fairly regular periods. "With abnormal periods (meaning unpredictable and missing months at a time), these calculators are not as reliable," notes Twogood.

I so loved comparing my baby to fruits and vegetables throughout pregnancy. She's an avocado. Wait, now she's four tangerines! Seriously though, it really helps you visualize what's going on inside your body. The produce comparisons only stop making sense in the final weeks, when your little one starts kicking like crazy. As far as I know, most fruit don't punch you in the bladder for no reason.

3With A Home Pregnancy Test

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Another way to get a due date at home? Check those home pregnancy tests.

"Most urine pregnancy tests should be positive by the fifth day of a missed period, when the gestational age of the pregnancy is between four-to-five weeks. If a woman is checking pregnancy tests and they are consistently negative, and the next day it is positive, she can guess she is around four-to-five weeks pregnant," Twogood explains.

Of course, any due date you estimate at home can't be considered official. You'll need to see a doctor or a midwife to get a date on the record, and it's really important to see them sooner rather than later, for the reasons discussed below.

4With An Early Ultrasound

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An accurate due date helps doctors monitor your baby's growth, and the earlier you receive an ultrasound, the more accurate your date will be. An ultrasound can also give you and your OB-GYN a heads up on an ectopic pregnancy, a miscarriage, twins, or multiples. "There are many reasons to seek prenatal care early in pregnancy," says Twogood. A proper due date is just one. Home calculations are all great fun, but you should get yourself to a doctor or midwife as soon as you know you're pregnant.

5Your Last Menstrual Period Combined With An Ultrasound

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"The most reliable due date is an ultrasound done in the first trimester that is consistent with dating from the first day of a woman's last menstrual period (LMP)," explains Twogood. "For example — a patient comes to me at seven weeks pregnant by her LMP and on ultrasound her fetus is measuring seven weeks. This is strong dating criteria."

For many reasons, it's good idea to see a doctor as soon as you know or suspect a pregnancy. Getting an accurate due date — confirmed by an ultrasound — is one of them. Next step: stocking up on prenatal vitamins for a 40 week haul, while planning the Best. Baby Shower. Ever.

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