From the first day of the last menstrual period, the standard estimate is 280 days, which is 40 weeks. From conception or ovulation, the matching estimate is 266 days, which is 38 weeks. The 14-day gap is the time the 28-day model puts between the period and ovulation.
Both numbers are the due-date convention. They are not a promise of the birth day. A commonly cited figure is that only a small share of births fall on the exact date. See how accurate due dates are.
The two counts
| Start | Days | Weeks | Example |
|---|---|---|---|
| Last period, 1 January 2026, 28-day cycle | 280 | 40 | 8 October 2026 |
| Same period, 32-day cycle | 284 | 40 weeks + 4 days | 12 October 2026 |
| Ovulation, 14 March 2026 | 266 | 38 | 5 December 2026 |
The cycle adjustment is one day for each day the cycle differs from 28. A 32-day cycle adds 4 days to the period-based date. You do not add those 4 days again if you already start from a known ovulation date.
The pregnancy due date calculator is the 280-day path. The due date by ovulation calculator is the 266-day path. How the due date is calculated shows Naegele’s rule, which is the same 280 days written as a calendar shortcut.
Labels around the due date
Clinicians group births by completed weeks from the last period. A common set of labels:
- Before 37 weeks: preterm
- 37 weeks 0 days through 38 weeks 6 days: early term
- 39 weeks 0 days through 40 weeks 6 days: full term
- 41 weeks 0 days through 41 weeks 6 days: late term
- 42 weeks 0 days and later: post-term
Those are category names used in prenatal care. They are not instructions about induction or delivery. Thirty-seven weeks from the period is 259 days, which is earlier than the 280-day due date on purpose.
Trimester dates split the same 40 weeks into three blocks. The week calculator tells you where today sits inside them.