How to use
Enter the first day of your last menstrual period and your usual cycle length. You’ll see your estimated due date, how many weeks and days pregnant you are, your trimester and an estimated conception date. You can also check how far along you’ll be on any other date.
If you know the conception date or conceived through IVF, choose how to calculate in the first field. Birth comes on average 266 days after conception, 261 days after a day-5 embryo transfer and 263 days after a day-3 transfer. Below the result you’ll see the current week and key dates: end of the first trimester, start of the third, the start of early term (37 weeks), the due date and 42 weeks.
How it’s calculated
It uses Naegele’s rule: 280 days (40 weeks) are added to the first day of your last period. If your cycle is longer or shorter than 28 days, the date shifts by the difference. Pregnancy is counted in gestational weeks — from the first day of your last period, not from conception.
Example
Last period started on March 1, 2026, with a 28-day cycle:
- estimated due date: December 6, 2026;
- on May 10, 2026 you’re 10 weeks pregnant, in the first trimester;
- estimated conception: around March 15.
With a 32-day cycle, the due date moves to December 10.
FAQ
How accurate is the due date? Only about 4–5% of babies are born on their due date; it’s an estimate, not a prediction of the day of birth. Your maternity care team determines pregnancy dating and follow-up; first-trimester ultrasound dating is more accurate and may differ from a calculation based on your last period.
How are gestational weeks different from weeks since conception? Gestational age is about 2 weeks more, because it’s counted from the start of your last cycle.
What if I don’t remember the date of my last period? Your doctor or midwife can date the pregnancy with an ultrasound.
This calculator is for information only and doesn’t replace prenatal care.