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Countdown to baby: How your pregnancy is dated

As you might imagine, knowing the exact day that you conceived your baby can be tricky. This may leave you scratching your head and asking, "How do we know the due date then?" Here's your answer:

Providers figure out your due date by using the first day of your last menstrual period (LMP). It's far easier for women to remember when their last period was than to guess the time of conception. Providers then add 40 weeks to your LMP to arrive at baby's due date. 

Understanding pregnancy weeks 

This means that providers count the first couple of weeks before you're even with child as part of your 40 weeks of pregnancy. It also means that when you say you're 10 weeks pregnant, your baby is only about eight weeks in the making. 

We know it's confusing, but this approach helps produce a more reliable due date. And while most babies don't arrive right on time, due dates are still an important guide for checking your baby's growth as your pregnancy progresses. 

How to calculate your due date

Want to calculate your due date yourself? It's simple—just take the date of your last menstrual period and add 40 weeks (280 days). For example, if that date were Jan. 1, your due date would be 40 weeks later on Oct. 8.

You can also use this due date calculator.

What happens when memory fails

Finding your due date can be difficult if your periods are irregular. Or you might not remember the first day of your last period. In these cases, your provider can estimate your due date by performing an ultrasound exam, which measures the length of the embryo or fetus.

Babies can be measured this way very early. But the best time is between the 8th and 18th week of pregnancy.

Using tests to find your due date is harder later on in pregnancy. That's just one more reason to start prenatal care early.

More pregnancy news

Were you a regular smoker or drinker before you discovered you're pregnant? Don't forget you have to quit in order to have a healthy baby. Learn more about the effects of smoking and drinking on your pregnancy

Sources: American College of Obstetricians and Gynecologists; American Pregnancy Association; March of Dimes

Reviewed 11/10/2023

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