Your due date is the date you are expected to reach 40 weeks of pregnancy. Doctors count from the first day of your last period, so the date can be worked out with simple arithmetic. This guide shows each method with worked examples. You can check any answer with our free pregnancy calculator.
Add 280 days (40 weeks) to the first day of your last menstrual period. By hand: add one year, subtract three months, add seven days. A period starting January 10, 2026 gives a due date of October 17, 2026.
Naegele's rule: due date from your last period
Example: your last period began on January 10, 2026.
- Take the first day of the period: January 10, 2026.
- Add one year: January 10, 2027.
- Subtract three months: October 10, 2026.
- Add seven days: October 17, 2026.
This matches adding 280 days directly. The rule assumes a 28-day cycle with ovulation around day 14.
Adjusting for cycle length
If your cycle is not 28 days, ovulation falls earlier or later, so move the due date by the difference between your cycle and 28 days.
| Cycle length | Adjustment | Due date (LMP January 10, 2026) |
|---|---|---|
| 24 days | -4 days | October 13, 2026 |
| 26 days | -2 days | October 15, 2026 |
| 28 days | +0 days | October 17, 2026 |
| 30 days | +2 days | October 19, 2026 |
| 32 days | +4 days | October 21, 2026 |
| 35 days | +7 days | October 24, 2026 |
Due date from your conception date
If you know the day you conceived, add 266 days (38 weeks). Conception normally happens about 14 days after the start of the period in a 28-day cycle, and 280 โ 14 = 266.
Example: conception on January 24, 2026 gives a due date of October 17, 2026.
Due date for IVF pregnancies
With IVF the age of the embryo is known, so dating uses the transfer date.
| Embryo transferred | Add to transfer date | Example (transfer March 1, 2026) |
|---|---|---|
| Day-3 embryo | 263 days | November 19, 2026 |
| Day-5 blastocyst | 261 days | November 17, 2026 |
| Day-6 blastocyst | 260 days | November 16, 2026 |
Your clinic will give the official date.
Dating with an ultrasound
If you do not know your LMP, or your cycles are irregular, an early ultrasound sets the date. Subtract the gestational age on the scan from the scan date to find a "start date", then add 280 days. For example, a scan on March 1, 2026 that measures 8 weeks 0 days gives a start date of January 4 and a due date of October 11, 2026.
ACOG guidance (Committee Opinion 700) says a first-trimester scan is the most accurate way to date a pregnancy. The LMP-based date is usually kept unless the scan differs by more than 5 days before 9 weeks, or more than 7 days from 9 weeks to 13 weeks 6 days.
Calculating how many weeks pregnant you are
Example: an LMP of January 10, 2026 and a date of April 14, 2026 is 94 days, which is 13 weeks and 3 days.
Pregnancy is counted from the LMP, so about two weeks pass before conception. That is why you are already "4 weeks pregnant" around the time of a missed period.
Trimesters and full term
| Stage | Weeks |
|---|---|
| First trimester | 0 to 13w6d |
| Second trimester | 14w0d to 27w6d |
| Third trimester | 28w0d until birth |
| Early term | 37w0d to 38w6d |
| Full term | 39w0d to 40w6d |
| Late term | 41w0d to 41w6d |
| Post-term | 42w0d or more |
Term definitions follow ACOG Committee Opinion 579.
How accurate is a due date?
A due date is an estimate. Only about 5% of babies are born on it, and most arrive between 37 and 42 weeks. Cycle length, ovulation timing, the baby's growth and multiples all change when labor starts. Think of the date as the middle of a window and follow your provider's advice.
Using the pregnancy calculator
- Pick a method: LMP, conception, IVF transfer, a known due date or ultrasound.
- Enter the date and any extra details, such as cycle length or scan weeks.
- Read the results: due date, current week and day, trimester, days to go, estimated conception and key dates.
- Open the week-by-week calendar to see the dates for every week, with your current week highlighted.
- Save it: copy the result, copy a link that keeps your dates, or add the due date to your calendar.
Your dates stay in your browser and are not sent anywhere.
๐ถ Free Pregnancy Calculator
Due date from LMP, conception, IVF or ultrasound, with week, trimester and a week-by-week calendar.
Open the Pregnancy Calculator โThis guide is general information, not medical advice. Your doctor or midwife confirms your due date.
Frequently asked questions
Conclusion
Your due date is an estimate: 280 days from the first day of your last period, adjusted for cycle length, or worked out from conception, an IVF transfer or an early ultrasound. Use the free pregnancy calculator to see your due date, week and trimester, and confirm the date with your doctor or midwife.