How due dates are estimated
The standard method (Naegele's rule) adds 280 days, or 40 weeks, to the first day of your last period. If your cycle is longer or shorter than 28 days, the date moves by the difference.
Trimesters
| Trimester | Weeks |
|---|---|
| First | Weeks 1 – 13 |
| Second | Weeks 14 – 27 |
| Third | Week 28 to birth |
Worked example
If your last period started on March 1 and your cycle is 28 days, add 280 days to get an estimated due date of December 6. With a 31-day cycle, ovulation likely happened about three days later, so the estimate moves to December 9.
Term definitions
| Term | Weeks of pregnancy |
|---|---|
| Early term | 37 weeks 0 days – 38 weeks 6 days |
| Full term | 39 weeks 0 days – 40 weeks 6 days |
| Late term | 41 weeks 0 days – 41 weeks 6 days |
| Postterm | 42 weeks 0 days and beyond |
These definitions come from the American College of Obstetricians and Gynecologists.
Frequently asked questions
How many babies arrive on their due date?
Only a small share. Most births happen within about two weeks before or after the estimated date.
Why count from my last period instead of conception?
The date of the last period is easier to know. Conception usually happens about two weeks later, which is why pregnancy is counted as 40 weeks.
What if I don't know the date of my last period?
An ultrasound in the first trimester can date a pregnancy accurately. Your healthcare provider can estimate the due date from the baby's measurements.