Track your baby's development, week by week



How is a pregnancy due date calculated?


Very few expectant moms know exactly when they conceived, however. Even if you only had sex once during your fertile period, you wouldn't necessarily conceive right then. It's possible that the sperm hung out in your fallopian tube for a few days until your egg came along, at which point it fertilized the egg and your baby was conceived.


Most people just estimate. The method typically used by healthcare providers is simply to count from the first day of your last period. For women with a menstrual cycle of average length, that day is usually about two weeks before conception, which explains why pregnancies are said to last 40 weeks. This method doesn't take into account how long your menstrual cycle actually is or when you think you might have conceived.

Due Date Calculator


BabyCenter's Due Date Calculator uses the same approach that your provider is likely to use. It gives you a due date that's 40 weeks from the first day of your last period.

If you do happen to know precisely when you conceived – perhaps you were tracking your ovulation and you know on which day your egg must have been fertilized – we give you the option of calculating your due date based on your conception date. Your result will be 38 weeks from that date.

Your healthcare provider might revise your due date if she has reason to believe it's off target. She'll measure your baby during an ultrasound exam, and if your baby is much bigger or smaller than would be reasonable given your due date, she'll provide you with a new due date.

(For a fun read, see our stories from parents who know when and where they did the deed that resulted in their child.)

Of course, a due date calculation is always approximate, whether it's from our tool or from your doctor or midwife. Only 1 in 20 women delivers on her due date. You're just as likely to go into labor any day during the two weeks before or after.


> Want more information about how the weeks, months, and trimesters of pregnancy are counted? See our pregnancy timing chart.


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