How is a due date calculated?
Your estimated due date is the first day of your last period plus 280 days. This is called Naegele's rule and assumes a regular 28 day cycle. A longer cycle means later ovulation, so the calculator moves the date later by the difference. A shorter cycle moves it earlier.
Example: if your last period started on March 1, add 7 days (March 8) and count back 3 months (December 8). Your due date is around December 8.
Which method should I choose?
- Last period: the quickest option if your cycles are regular and you remember the first day.
- Conception date: if you know the day you conceived from ovulation tracking. Pregnancy is dated from 2 weeks before conception.
- IVF transfer: the most precise, since the transfer day is known. Choose a day 3 or day 5 embryo.
- Ultrasound: enter the weeks and days your provider measured. First trimester scans are the most accurate.
How are pregnancy weeks counted?
Pregnancy weeks are counted as completed weeks. At "12 weeks 5 days" you are in week 12, and you move to week 13 two days later. The first trimester runs to week 13, the second from 14 to 27 and the third from 28 until birth. See the week by week pregnancy guide for details on every week, and the pregnancy weight gain calculator to check your gain.
Frequently asked questions
How is a due date calculated?
The most common method is Naegele's rule: add 280 days, or 40 weeks, to the first day of your last menstrual period. If your cycle is longer or shorter than 28 days, the date shifts by the difference. An early ultrasound can refine it.
Why is pregnancy counted from the last period?
The exact day of conception is rarely known, but most people remember when their last period started. So pregnancy is dated from that day, which means the first two weeks happen before you actually conceive. Ovulation usually happens around the end of week 2.
Will I give birth on my due date?
Only about 4 to 5 percent of babies arrive on their exact due date. Birth any time between 37 and 42 weeks is considered normal. Think of the date as a planning point for your care, not a deadline.
What if my ultrasound date is different?
A first trimester ultrasound measuring crown to rump length is the most accurate way to date a pregnancy. If it differs from your period date by more than about a week, your provider will usually go with the ultrasound date.
How is an IVF due date calculated?
With IVF the transfer date is known exactly. For a day 5 embryo (blastocyst) transfer, add 261 days to the transfer date. For a day 3 embryo transfer, add 263 days.
