Your due date and how far along you are

Pregnancy due date

Estimate your baby's due date and current week of pregnancy from the first day of your last menstrual period.

Formula

due date = first day of last menstrual period + 280 days (40 weeks), per Naegele's rule

Assumptions

• Naegele's rule assumes a regular 28-day menstrual cycle — if yours is longer or shorter, your actual due date may differ from this estimate.
• Only a small share of babies — commonly cited as under 5% — arrive exactly on the estimated due date. Birth anywhere from 37 to 42 weeks is considered term.
• This is informational only, not medical advice — your healthcare provider's ultrasound-based estimate is more accurate than a calendar calculation.

Frequently asked questions

What's Naegele's rule, and how accurate is it?
It's the standard obstetric estimate: add 280 days (40 weeks) to the first day of your last period. It's a widely used starting point, but only a small fraction of births happen exactly on that date — delivery anywhere between 37 and 42 weeks is normal.

What if my cycle isn't 28 days?
This calculator assumes a standard 28-day cycle. If yours runs longer or shorter, your true ovulation and conception date shifts accordingly, so your actual due date may be earlier or later than this estimate — your doctor's ultrasound dating will account for this.

What counts as "full term"?
Obstetric guidance splits the term window rather than treating it as one block: 37 0/7 to 38 6/7 weeks is early term, 39 0/7 to 40 6/7 is full term, 41 weeks is late term, and 42 weeks or beyond is postterm. Births across that whole 37-42 week range are normal — this calculator gives the 40-week date, not a precise prediction.

Sources

• StatPearls — Estimated Date of Delivery (Naegele's rule)