How is the due date calculated?
The most common method (Naegele’s rule) adds 280 days (40 weeks) to the first day of the last menstrual period, assuming a 28-day cycle with ovulation around the middle. If your cycle is longer or shorter, the date is adjusted by the difference.
The four methods
| What you know | How the due date is calculated |
|---|---|
| First day of last period | + 280 days, plus (or minus) the difference between your cycle and 28 days |
| Ovulation / conception date | + 266 days (38 weeks) |
| Day-5 embryo transfer | + 261 days |
| Day-3 embryo transfer | + 263 days |
| Ultrasound scan | Scan date + (280 days − pregnancy age at the scan) |
Whatever the method, the pregnancy age is shown the standard medical way: counted from the first day of the last period, so you are about “2 weeks pregnant” at ovulation. This is the number doctors write on reports.
The three trimesters
| Trimester | Weeks |
|---|---|
| First | Up to 13 weeks and 6 days |
| Second | From 14 completed weeks to 27 weeks and 6 days |
| Third | From 28 completed weeks to birth |
Why the actual date may differ
Only about 5% of births happen exactly on the estimated date; most occur within two weeks before or after, which is completely normal. An early ultrasound usually gives the most accurate estimate, and your doctor may adjust the date based on it.
Frequently Asked Questions
What if my periods are irregular?
Then this method is less accurate, and an early ultrasound, which measures the baby directly, gives a better estimate of how far along you are.
Can I calculate from the date of ovulation or conception?
Yes. Choose “Ovulation / conception date” and 266 days (38 weeks) are added to it. If you know your ovulation day precisely, for example from ovulation tests, this is more accurate than using your period.
How is the due date calculated after IVF?
With IVF the date of conception is known exactly, so the due date is counted from the transfer: + 261 days for a day-5 embryo (blastocyst) or + 263 days for a day-3 embryo. Choose “IVF embryo transfer” and select the embryo’s age.
My ultrasound gave a different date than my period. Which one counts?
Doctors usually go by the first ultrasound (especially in the first trimester) when it differs noticeably from the period-based date, because ovulation can happen later than expected. Choose “Ultrasound scan” and enter the scan date and the pregnancy age from the report. Your doctor has the final say.
Which month of pregnancy am I in?
Use our pregnancy weeks to months calculator to see your month and trimester from your weeks.
I’m trying to conceive. How do I find my fertile days?
Use our ovulation and fertile days calculator to see your expected ovulation day and the best days to conceive.