How to use the Pregnancy Due Date Calculator
- Choose whether you know the first day of your last period or the conception date.
- Pick the date.
- If using your period, enter your usual cycle length (28 days is typical).
- Read your due date, current week and trimester dates.
Formula used
Trimesters: first = weeks 1–13, second = weeks 14–27, third = week 28 to birth.
Worked examples
| Last period | Cycle | Due date |
|---|---|---|
| 1 January 2026 | 28 days | 8 October 2026 |
| 15 March 2026 | 30 days | 22 December 2026 |
| 1 June 2026 | 28 days | 8 March 2027 |
Dating scan vs due date calculator
An early ultrasound scan (usually between 8 and 14 weeks) is the most accurate way to date a pregnancy, and doctors may change your due date after it. Use this calculator for planning, and follow the date your doctor or midwife gives you. Counting days to an appointment? The Date Difference Calculator can help.
Medical note: this tool gives general estimates for healthy adults. It is not a diagnosis. Talk to a doctor or registered dietitian before making big changes, especially if you are pregnant, under 18, an athlete or living with a health condition.
Frequently asked questions
How accurate is a due date?
It is an estimate. Most babies are born between 38 and 42 weeks, and only about 4% to 5% arrive on the exact day.
How many weeks is a full-term pregnancy?
Full term is 39 weeks 0 days to 40 weeks 6 days. Babies born from 37 weeks are not considered premature.
Why are weeks counted from the last period?
Because most people know that date, while the exact conception date is usually unknown. Conception usually happens about two weeks after the period starts.
Can I use this for IVF?
For IVF, add 266 days to the egg retrieval date, or follow the date from your clinic.
What if my periods are irregular?
The calculator is less reliable. An early dating scan gives a better estimate.
Sources and references
Our team builds and tests every tool by hand, checks each formula against official sources (IRS, HMRC, the Income Tax Department of India, CBSE, NHS, WHO) and reviews pages whenever rules change. Last reviewed on . Spotted a mistake? Tell us.