Pregnancy Due Date Calculator
Enter the first day of your last menstrual period to get the estimated due date, how many weeks along you are today, and your trimester dates.
Nothing you enter is sent anywhere — the calculation happens in your browser.
How the math works
This is Naegele's rule, the method used by obstetric clinics worldwide: pregnancy is counted as 40 weeks from the last menstrual period, because that's the date people know most reliably — ovulation (and conception) happened about two weeks later, and the estimated conception date is shown as roughly 266 days before the due date. If your cycle runs longer or shorter than 28 days, ovulation day shifts and so does the estimate, by about a day per day of difference.
Reading the result
Pregnancy is divided into first trimester (weeks 1–13), second (14–27) and third (28–40+). "Full term" now means between 39 weeks 0 days and 40 weeks 6 days — only around 1 in 20 babies arrives exactly on the estimated day, with most healthy births landing somewhere in weeks 37–42. Your first ultrasound is the best dating tool; if it disagrees with this estimate by more than about a week, your clinician will usually go with the scan.
FAQ
- I conceived by IVF — should I use this?
- No. For IVF, due dates are calculated from the transfer date (embryo age is known exactly); your fertility clinic will give you the date directly.
- My cycles are irregular — can I trust this?
- Less so. With irregular cycles the LMP method is unreliable and an early ultrasound is the right way to date the pregnancy.
- What counts as the first day of the last period?
- Day 1 of your last full menstrual bleeding — not spotting, and not the day it ended.
This tool provides general information only and is not medical advice. Confirm your dates with a qualified healthcare professional.