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 |
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.