Pregnancy Due Date Calculator
Your estimated due date is 280 days (40 weeks) after the first day of your last period. For example, a last period starting March 1 gives a due date of about December 6.
Pick a date above to see your due date, week and trimester.
Estimates only, not medical advice. Only about 1 in 20 babies arrives on the due date. Your doctor or midwife may adjust your dates after an ultrasound.
How your due date is calculated
Doctors count pregnancy from the first day of your last menstrual period (LMP), even though conception happens about two weeks later. A full-length pregnancy is about 280 days, or 40 weeks, from that date. The calculator adds 280 days to your LMP, which matches Naegele's rule (add one year, subtract three months, add seven days) to within a day or two.
The standard rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation and conception probably happened earlier or later, so the calculator shifts the date by the difference. A 32-day cycle moves your due date four days later.
If you know your conception date, the due date is 266 days (38 weeks) after it. For IVF, the embryo's age is taken into account: a day-5 blastocyst transfer adds 261 days and a day-3 embryo transfer adds 263 days. All four methods land on the same date when the inputs match, and you can watch the working in the Show the math panel.
Weeks, trimesters and term
Pregnancy is described in completed weeks and days, so 13 weeks and 2 days means 13 full weeks plus 2 days. The first trimester runs to 13 weeks and 6 days, the second from 14 weeks to 27 weeks and 6 days, and the third from 28 weeks until birth.
Doctors describe the timing of birth as early term (37 weeks to 38 weeks and 6 days), full term (39 weeks to 40 weeks and 6 days), late term (41 weeks to 41 weeks and 6 days) and post term (42 weeks or more). Most babies arrive within two weeks either side of the due date.
How accurate is a due date?
A due date is a well-informed estimate, not a deadline. Only about 1 in 20 babies is born on the day itself. Your doctor or midwife may change your dates after an early ultrasound, especially if the scan measurement differs noticeably from your last period dating. Always follow their advice over any online calculator.
Frequently asked questions
- How do I calculate my due date from my last period?
- Add 280 days (40 weeks) to the first day of your last period. If your cycle is not 28 days, add or subtract the difference first. Enter the date above to see each step.
- How many weeks pregnant am I if I just conceived?
- About two weeks. Pregnancy weeks are counted from the first day of your last period, which is roughly 14 days before conception in a 28-day cycle.
- What if my cycle is not 28 days?
- Enter your average cycle length. A longer cycle pushes the due date later and a shorter cycle pulls it earlier, because ovulation moves with the cycle.
- How accurate is my estimated due date?
- Only about 5% of babies are born on the due date. Most arrive between 37 and 42 weeks. Treat the date as a guide.
- Can an ultrasound change my due date?
- Yes. Your doctor or midwife may adjust it if an early scan measures noticeably different from your last-period dating.