Pregnancy due date calculator
Enter the first day of your last period, or your conception date if you know it, to see your estimated due date and current week of pregnancy.
An estimate only. Your doctor confirms the date with an ultrasound.
Enter the date to see the result.
How the due date is worked out
The standard method adds 280 days, which is 40 weeks, to the first day of your last period. If your cycle is longer or shorter than 28 days, the date moves by the difference, because ovulation moves with it. From a conception date the due date is 266 days later, and for IVF it is 261 days after a day 5 embryo transfer or 263 days after a day 3 transfer.
For example, a last period starting on 1 March 2026 gives a due date of 6 December 2026. With a 35 day cycle the same period gives 13 December.
What can change your due date
The date is an estimate, and a few things can move it.
- An uncertain or forgotten last period date
- Irregular cycles, so ovulation did not happen when expected
- An early ultrasound that measures differently from your dates; your doctor may then change the date
- A multiple pregnancy, which often ends a little earlier
Reading the result
Think of the due date as the middle of a window. A birth from 37 weeks is considered term, and few babies arrive exactly on the day. Use the check-up schedule below the result to plan your visits, add the dates to your calendar, and tick off the stage checklist as you go. If your doctor gives you a different date, follow your doctor.
How to use it
- Choose how to calculate: from the first day of your last period, or from the conception date.
- Enter the date, and adjust the cycle length if yours is not 28 days.
- Your due date, current gestational age, trimester and days remaining appear instantly.
Frequently asked questions
How is the due date calculated?
With Naegele's rule: add 280 days (40 weeks) to the first day of your last period. If your cycle is longer or shorter than 28 days, the result shifts by the difference.
How do I calculate it from the conception date?
Add 266 days (38 weeks) to the conception date. This is useful when the date is known exactly, for example after IVF.
How accurate is the due date?
It is an estimate, not a fixed date. Many babies arrive some weeks before or after it. Your doctor normally confirms the date with an ultrasound, based on measurements of the baby rather than on this calculator.
When does each trimester start?
This calculator uses the common split: first trimester through week 13, second from week 14 to 27, third from week 28 until birth.
Is my data sent anywhere?
No. Everything is calculated in your browser and no date is sent to a server.
Sources
- Methods for Estimating the Due DateACOG (American College of Obstetricians and Gynecologists)
- New guidelines on antenatal care for a positive pregnancy experience (2016)WHO (World Health Organization)
- Tdap Vaccination for Pregnant WomenCDC (US Centers for Disease Control and Prevention)
Based on the sources above. This is general information, not medical advice. Ask your doctor about your own situation.