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Due Date Calculator: Pregnancy Weeks and Trimesters
Estimate your due date from your last period and see how many weeks pregnant you are, your trimester and key dates. Adjusted for cycle length.
Updated: August 27, 2026 · 🔒 Your data never leaves your browser
The estimated due date (EDD) uses Naegele's rule: the first day of your last period plus 280 days (40 weeks) for a 28-day cycle. A longer or shorter cycle shifts the date by the difference. Pregnancy is also dated from the last period, so 'gestational' weeks run about two weeks ahead of weeks from conception. On accuracy: only about 4% give birth exactly on the due date, and 37–42 weeks is considered normal.
Pick the date of your last period — we'll estimate the due date and how far along you are.
How the due date is calculated
The basis is Naegele's rule: add 280 days — 40 weeks — to the first day of your last period. To do it in your head: subtract 3 months and add 7 days to the last period date.
The rule assumes a regular 28-day cycle. If your cycle is longer or shorter, ovulation happens later or earlier, so the calculator shifts the due date by the difference for a closer estimate.
Why it is an estimate
Only about 4% of babies are born exactly on the due date. A span of 37 to 42 weeks is considered normal — a few weeks either way is normal variation, not a problem. Length is influenced by age, number of previous births and individual factors.
The most accurate dating comes from a first-trimester ultrasound (before 13–14 weeks) by fetal size. If it differs from the period-based date by more than a few days, the doctor uses the ultrasound date.
Key milestones
| Stage | What it means |
|---|---|
| 14 weeks | Second trimester begins |
| 24 weeks | Fetal viability threshold |
| 37 weeks | Pregnancy is full term |
| 40 weeks | Estimated due date (EDD) |
| 42 weeks | Post-term pregnancy |
Читайте также: The Menstrual Cycle: Phases, Hormones and How It Affects You
When to trust the estimate with caution
With an irregular cycle, ovulation shifts unpredictably and period-based dating becomes rough. The same applies to a cycle much shorter or longer than average. In those cases ultrasound sets the due date and gestational age more reliably, and the calculator is only a guide.
This tool does not replace medical care: it estimates dates but does not monitor the pregnancy.
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FAQ
How do I calculate my due date from my last period?+
By Naegele's rule: add 280 days (40 weeks) to the first day of your last period. A quick mental shortcut is to subtract 3 months and add 7 days. This assumes a regular 28-day cycle; a different cycle shifts the date by the difference. The calculator does this automatically.
Why is pregnancy dated from the period, not conception?+
It is the obstetric convention: the conception date is usually not known precisely, but the first day of the last period is remembered. So the 'gestational' age runs about 2 weeks ahead of the 'embryonic' age from conception. When people say '40 weeks pregnant,' they mean gestational weeks from the last period.
How accurate is the estimated due date?+
It is an estimate, not an exact day. Only about 4% of babies arrive exactly on the due date; 37–42 weeks is considered normal. A first-trimester ultrasound (before 13–14 weeks) dates the pregnancy most accurately by fetal size — if it differs from the period-based date by more than a few days, the doctor uses the ultrasound date.
What if my cycle is irregular?+
With an irregular cycle, period-based dating is less reliable because ovulation shifts. In that case the due date and gestational age are set more accurately by ultrasound. The calculator lets you set cycle length, but with marked irregularity treat the result as a rough guide.
What do the weeks and trimesters mean?+
Pregnancy is divided into three trimesters: the first up to 14 weeks, the second 14–27, the third from 28 weeks. Key milestones: 24 weeks is the viability threshold, from 37 weeks the pregnancy is full term, and after 42 it is post-term. The calculator shows your current stage and the dates of these milestones.
Can the calculator tell the baby's sex or the exact conception date?+
No. It estimates the due date, gestational age and an approximate conception (ovulation) date — but not the baby's sex or the exact day of conception. Sex is determined by second-trimester ultrasound or a blood test, and only an approximate ovulation window can be inferred, not an exact conception date.
References
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