Due Date Calculator
Find your due date from your last period, conception or IVF transfer
Your details
28 days is typical. Longer cycles push ovulation, and the due date, later.
Your pregnancy
Enter a date to see your due date and milestones.
Frequently asked questions
How is a due date calculated?
The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. That assumes a 28 day cycle with ovulation on day 14, so this calculator adds or subtracts days when your cycle is longer or shorter. From a known conception date the due date is 266 days later.
How do you calculate an IVF due date?
Count from the transfer date and account for the embryo's age. For a day 5 blastocyst the due date is 261 days after the transfer; for a day 3 embryo it is 263 days. Fertility clinics use the same arithmetic, so IVF due dates tend to be very precise.
How accurate is a due date?
It is an estimate. Only about 4% of babies are born on the exact day, and most arrive within two weeks either side of it. A first trimester ultrasound measuring the embryo is the most accurate way to date a pregnancy, and your doctor may move the date if the scan disagrees by more than a few days.
Why does pregnancy count from the last period and not conception?
Most people know the date of their last period but not when they conceived, so obstetrics counts gestational age from the last period. That is why you are considered 2 weeks pregnant around conception, and why a pregnancy lasts 40 weeks rather than 38.
When do the trimesters start and end?
The first trimester runs to the end of week 13 (13 weeks 6 days), the second from week 14 to the end of week 27, and the third from week 28 until birth. Full term is 39 weeks 0 days to 40 weeks 6 days.