Pregnancy & Fertility
Due Date Calculator
Forty weeks from your last period, that's the classic due date estimate. See the date, trimester milestones, and full-term window.
The estimated due date is calculated with Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period. It's an estimate — most babies arrive between 37 and 42 weeks. Dates are estimates — confirm timing with your provider and ultrasound where relevant.
Due date = LMP + 280 days.
Only ~4–5% of babies arrive on the date.
Full term: 39–40 weeks.
Calculate Your Due Date
Estimated Due Date
What Is Due Date?
Pregnancy is dated as 40 weeks from the first day of the last menstrual period (LMP), even though conception occurs about two weeks later.
The due date is an estimate: only 4–5% of babies arrive exactly on it. Early ultrasound dating in the first trimester is more accurate and may adjust the date.
How It Is Calculated
Due date = LMP + 280 days (Naegele’s rule)
Or: conception date + 266 days
Example: Example: LMP Jan 10 → estimated due date Oct 17
Pregnancy milestones
| Milestone | When |
|---|---|
| Heartbeat visible on ultrasound | 6–7 weeks |
| First trimester ends | 13 weeks |
| Anatomy scan | 18–22 weeks |
| Full term | 39–40 weeks |
| Post-term threshold | 42 weeks |
Limitations
- Estimates assume a 28-day cycle, longer cycles shift the date.
- Early ultrasound is the gold standard for dating.
- This tool is educational; your provider's dating takes precedence.
Sources & Review
References used for this calculator’s formulas and thresholds:
Due Date FAQ
How accurate is the due date?
Calendar estimates are ±1–2 weeks; first-trimester ultrasound dating narrows this to about ±5 days.
Why count pregnancy from the last period?
Because most people know that date but not the conception date. The 40 weeks include the two weeks before ovulation.
What if I have long cycles?
Ovulation, and conception, happen later, so the calendar overestimates. Ultrasound dating corrects this.
How far past the due date is safe?
Pregnancy is 'post-term' after 42 weeks. Monitoring usually intensifies after 41 weeks.
Important disclaimer: Results are educational estimates, not a substitute for professional medical advice. Discuss your numbers with a clinician before acting on them.