How the due date is calculated
The calculator uses Naegele's rule, the standard method used by midwives and doctors: the due date is 280 days (40 weeks) after the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14, so the calculator moves the date by the difference if your cycle is longer or shorter.
Due date = first day of last period + 280 days + (cycle length − 28)
If you know the conception date, the due date is 266 days (38 weeks) after it.
Worked example: a last period starting on 1 March 2026 with a 28-day cycle gives an estimated due date of Sunday 6 December 2026. With a 30-day cycle it moves two days later, to 8 December.
Trimesters and pregnancy weeks
| Stage | Weeks |
|---|---|
| First trimester | Week 1 to the end of week 13 |
| Second trimester | Week 14 to the end of week 27 |
| Third trimester | Week 28 to birth |
| Full term | 39 weeks 0 days to 40 weeks 6 days (ACOG definition) |
Pregnancy weeks are counted from the first day of the last period, about two weeks before conception. Definitions of trimesters vary slightly between health services.
Your due date is an estimate. Only a small share of babies arrive on the exact day. Most are born between 37 and 42 weeks. In the UK and the Netherlands, a dating scan at around 10 to 14 weeks is used to confirm the date; in the US and Canada, an early ultrasound is used in the same way.
Medical note: this calculator gives general information only and does not replace care from a midwife, verloskundige, obstetrician or doctor. If you think you might be pregnant, contact your GP, family doctor, midwife or huisarts.
Sources
- NHS, Pregnancy due date calculator and Your pregnancy and baby guide
- American College of Obstetricians and Gynecologists, Committee Opinion 700: Methods for estimating the due date
- ACOG Committee Opinion 579: Definition of term pregnancy
Frequently asked questions
How is a pregnancy due date calculated?
Add 280 days (40 weeks) to the first day of your last period, adjusted for cycle length. This is Naegele's rule, used by midwives and doctors.
How accurate is a due date calculator?
It gives an estimate. Most babies are born within a few weeks either side, between 37 and 42 weeks. A dating scan gives a more accurate date.
How many weeks pregnant am I?
Pregnancy is counted from the first day of your last period. Enter that date and the calculator shows how many weeks and days along you are today.
What if I don't know my last period date?
Use the conception date if you know it, or ask your midwife or doctor about a dating scan, which estimates the due date from the baby's size.