Naegele's rule
Due date = first day of last period + 280 days
Adjusted = + (cycle length − 28) for longer or shorter cycles
Conception ≈ last period + cycle length − 14
| Stage | Weeks | Notes |
|---|---|---|
| First trimester | 0 – 13 | Dating scan usually 8 to 14 weeks |
| Second trimester | 14 – 27 | Anomaly scan around 20 weeks |
| Third trimester | 28 – 40 | Full term from 37 weeks |
| Early term | 37 – 38 | |
| Full term | 39 – 40 | |
| Post term | 42+ | Induction usually discussed |
- Pregnancy is counted from the first day of your last period, not from conception — so you are already counted as two weeks pregnant at conception.
- 280 days is 40 weeks. Actual gestation from conception is about 38 weeks.
- Naegele's rule assumes a 28-day cycle; a longer cycle pushes the due date later, which this adjusts for.
- Only about 4% of babies are born on the estimated date; most arrive within two weeks either side.