Enter the first day your last period began.

Average cycle is 28 days. Adjust for your cycle.

Enter your estimated date of conception or positive ovulation test.

How Your Due Date Is Calculated

Our calculator uses Naegele's Rule — the gold standard in obstetrics. It adds 280 days (40 weeks) to the first day of your last menstrual period, adjusted for your personal cycle length.

If you use the conception date method, we add 266 days (38 weeks) from your estimated conception date, as fertilization typically occurs around ovulation.

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Naegele's Rule

The most widely used medical formula. LMP + 280 days = estimated due date.

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Cycle Adjusted

We adjust calculations for cycles shorter or longer than the standard 28 days.

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Trimester Dates

Get a full timeline including first, second, and third trimester start dates.

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Doctor Confirmed

Always confirm with your OB/GYN. An ultrasound gives the most accurate date.

Frequently Asked Questions

The most common method is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a standard 28-day cycle with ovulation on day 14. Our calculator also adjusts for custom cycle lengths.
Only about 5% of babies are born on their exact due date. Most arrive within 2 weeks before or after. The calculator gives a reliable estimate, but an early ultrasound (especially before 13 weeks) provides the most accurate due date.
Gestational age is counted from the first day of your last menstrual period — this is the age used by doctors. Fetal age (or embryonic age) counts from actual conception, which is typically about 2 weeks less than gestational age.
The first trimester runs from conception to the end of week 12. The second trimester spans weeks 13–26. The third trimester begins at week 27 and continues until birth, typically around week 40.
Use the Conception Date tab if you know when you ovulated or had a positive ovulation test. Otherwise, your doctor can perform an ultrasound to estimate gestational age and give you a more accurate due date.
Not exactly. Pregnancy is actually closer to 10 calendar months (40 weeks). This is because months have varying lengths, while weeks are exact. Doctors and midwives always refer to pregnancy in weeks for precision.
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