Quick answer: A due date is estimated with Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period, adjusting for cycles that are longer or shorter than 28 days. If the last period began on January 10, 2026, the estimated due date is Saturday, October 17, 2026.
Finding out you are pregnant brings a lot of questions, and “when is the baby due?” is usually the first. This calculator estimates your due date from the first day of your last menstrual period, and shows how far along you are, how many days remain, and when each trimester begins.
If your cycles are longer or shorter than 28 days, change the cycle length. The due date shifts to match, because ovulation, and so conception, happens later or earlier in the cycle.
How the due date is estimated
The calculator uses Naegele’s rule, the standard method for dating a pregnancy: a full-term pregnancy lasts about 280 days (40 weeks) from the first day of the last period, assuming a 28-day cycle with ovulation on day 14.
Conception ≈ first day of last period + (cycle length − 14) days
Pregnancy weeks are counted from the start of the last period, not from conception, which is why you are already “4 weeks pregnant” when you first miss a period. The trimesters are: first up to 13 weeks and 6 days, second from 14 weeks to 27 weeks and 6 days, and third from 28 weeks.
Only about 4% of babies are born on their due date, and most arrive between 37 and 42 weeks. Doctors often adjust the date after an early ultrasound, which can be more accurate than counting from the last period, especially if cycles are irregular.
A worked example
Example. If the first day of the last period was January 10, 2026 and the cycle is 28 days, the due date is 280 days later: Saturday, October 17, 2026. On April 18, 2026 the pregnancy would be exactly 14 weeks, the start of the second trimester.
Pregnancy stages by week
| Stage | Weeks of pregnancy |
|---|---|
| First trimester | Up to 13 weeks and 6 days |
| Second trimester | 14 weeks to 27 weeks and 6 days |
| Third trimester | 28 weeks onwards |
| Early term | 37 weeks to 38 weeks and 6 days |
| Full term | 39 weeks to 40 weeks and 6 days |
| Late term | 41 weeks to 41 weeks and 6 days |
| Post-term | 42 weeks and beyond |
Definitions used by the American College of Obstetricians and Gynecologists. The tool counts weeks from the first day of the last period.
Common mistakes to avoid
- Using the wrong start date. Use the first day of the last period, not the day you noticed a missed period or the date of conception.
- Forgetting to change the cycle length. A 35-day cycle moves the due date about a week later than a 28-day cycle.
- Treating the due date as the birth date. Only a small share of babies arrive on the day, and doctors often adjust the date after an early ultrasound.
Frequently asked questions
How accurate is a due date calculator?
It is an estimate. Only a small share of babies are born exactly on the due date. If your cycles are irregular or you are not sure of the date of your last period, an early ultrasound gives a better estimate.
What if I do not know my last period date?
Ask your doctor or midwife about an ultrasound dating scan. They can estimate the gestational age from the size of the baby.
How many weeks is a full-term pregnancy?
Full term is 39 to 40 weeks and 6 days. Early term is 37 to 38 weeks and 6 days, and post-term starts after 42 weeks.
When does the second trimester begin?
At 14 weeks, which is 98 days after the first day of your last period. The third trimester begins at 28 weeks, 196 days after.
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Sources and further reading
- Naegele's rule (Wikipedia)
- Gestational age (Wikipedia)
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date, 2017
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