How to find your due date
- Choose what you know: the first day of your last period, the conception date, or an IVF transfer date.
- Enter the date. For a period, add your usual cycle length if it isn't 28 days; for IVF, pick a day-3 or day-5 embryo.
- Read your estimated due date, how many weeks and days pregnant you are today, and your trimester.
- Use the key dates table to see when each trimester starts and when the pregnancy reaches term.
How the due date is worked out
- Last period:
due date = first day of last period + 280 days(Naegele's rule). For a cycle longer or shorter than 28 days, the date moves by the difference. - Conception:
due date = conception + 266 days, since ovulation is assumed 14 days into a 28-day cycle. - IVF:
transfer date + 263 daysfor a day-3 embryo, or+ 261 daysfor a day-5 embryo, as ACOG recommends.
Weeks of pregnancy are counted from 280 days before the due date, which is how doctors and midwives count them, so you're “2 weeks pregnant” around the time of conception.
Worked example
Last period started on 1 January 2026, with a 28-day cycle:
- 1 January 2026 + 280 days = Thursday, 8 October 2026.
- On 12 March 2026 that's 70 days, or 10 weeks and 0 days pregnant: the first trimester.
- With a 35-day cycle, the due date moves 7 days later, to 15 October 2026.
Trimesters and term
| Stage | Weeks of pregnancy |
|---|---|
| First trimester | Up to 13 weeks 6 days |
| Second trimester | 14 weeks 0 days to 27 weeks 6 days |
| Third trimester | 28 weeks onwards |
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Post-term | 42 weeks onwards |
Why your due date may change
A date worked out from a period assumes a regular cycle and a remembered start date. ACOG says an ultrasound measurement in the first trimester is the most accurate way to date a pregnancy: if a scan before 14 weeks differs from the period-based date by more than 7 days, the due date is moved to match the scan. Pregnancy normally lasts 37 to 42 weeks, so the due date is a guide, not an appointment.
Not medical advice
This calculator gives an estimate, not medical advice. Your midwife or doctor will confirm your due date, usually with a dating scan. If you have pain, bleeding or any worries during pregnancy, contact them straight away.
Sources
Each link was last checked on .
- Methods for Estimating the Due Date (Committee Opinion 700, reaffirmed 2025), American College of Obstetricians and Gynecologists (2017): the due date 280 days after the last period, 261 or 263 days after an IVF transfer, trimester boundaries, and early ultrasound as the most accurate dating.
- Definition of Term Pregnancy (Committee Opinion 579, reaffirmed 2025), American College of Obstetricians and Gynecologists (2013): early term from 37 weeks, full term from 39, late term from 41 and post-term from 42.
- Pregnancy due date calculator, NHS (2024): adjusting for cycles longer or shorter than 28 days, pregnancy lasting 37 to 42 weeks, and the 12-week scan dating more accurately.
Common uses
- Just found out you're pregnant. Enter the first day of your last period to see your estimated due date and how many weeks along you are before your first appointment.
- Planning leave and appointments. The key dates table shows when each trimester starts and when the pregnancy reaches full term, to plan work leave and travel.
- After an IVF transfer. Date an IVF pregnancy from the transfer day and embryo age, the way fertility clinics do, instead of from a period.