Pregnancy Due Date Calculator
Estimate your due date based on your last period. Enter the first day of your LMP and your average cycle length and you'll get an estimated due date, current gestational age, and which trimester you're in.
How to use
- Enter the first day of your last period.
- Enter your average cycle length in days. 28 is the default, adjust if yours is different.
- You'll see your estimated due date, how far along you are, and your current trimester.
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The standard way to calculate a due date is Naegele rule: take the first day of your last menstrual period (LMP), add 1 year, subtract 3 months, and add 7 days - or equivalently, add 280 days (40 weeks). This assumes a 28-day cycle with ovulation at day 14. For longer or shorter cycles, the estimated due date shifts accordingly.
Only about 5% of babies are actually born on the calculated due date. Most arrive within a 2-week window on either side. At your first prenatal appointment, an ultrasound in the first trimester can give a more accurate date based on the fetal measurements - especially if your cycles are irregular. The LMP date is a starting point, not a precise prediction.
Frequently Asked Questions
What is gestational age vs fetal age?
Gestational age counts from the first day of your last period - so you are considered 2 weeks pregnant even before conception occurs. Fetal age (conceptional age) counts from the actual day of conception, roughly 2 weeks later. Doctors and apps almost always use gestational age.
What are the three trimesters?
First trimester: weeks 1-12. Second trimester: weeks 13-26. Third trimester: weeks 27-40. The second trimester is when most women feel better as morning sickness eases. The third trimester is when fetal weight gain is most rapid.
Can an ultrasound change my due date?
Yes. A first-trimester ultrasound (before 14 weeks) is the most accurate way to date a pregnancy. If the ultrasound date differs by more than 5-7 days from the LMP date, your doctor may adjust the official due date.