The basic rule
Doctors date a pregnancy from the first day of your last menstrual period (LMP), not from conception, because that date is usually known while conception isn’t. A pregnancy lasts about 280 days, or 40 weeks, from that day.
Due date = first day of last period + 280 days
A shortcut called Naegele’s rule gives the same answer: add one year, subtract three months and add seven days. If your last period started on 1 January 2026, your due date is 8 October 2026.
The pregnancy due date calculator does this for you, and also shows how many weeks pregnant you are today and the dates of the usual scans.
If your cycle isn’t 28 days
The 280-day rule assumes a 28-day cycle with ovulation around day 14. With longer cycles, ovulation comes later, so the due date moves later too: a 35-day cycle moves it about a week. Enter your usual cycle length in the calculator for a better estimate.
Other ways to date a pregnancy
- Conception date known: add 266 days (38 weeks).
- IVF: for a day-5 embryo transfer, the due date is 261 days after the transfer; for a day-3 transfer, 263 days.
- Early ultrasound: the scan measures the baby and gives the pregnancy’s age in weeks and days. Count forward to 40 weeks from that.
Counting the weeks
Weeks are written as completed weeks plus days, such as 14+2 for 14 weeks and 2 days.
| Stage | Weeks |
|---|---|
| First trimester | Up to the end of week 13 |
| Second trimester | Weeks 14 to 27 |
| Third trimester | Week 28 to birth |
| Early term | 37 weeks |
| Full term | 39–40 weeks |
Key dates to plan around
Common checks include the dating and nuchal translucency (NT) scan at 11–13 weeks, the anatomy (anomaly) scan at 18–22 weeks and the glucose screening test at 24–28 weeks. Schedules differ by country and doctor, so follow your own care plan; the calculator simply shows when each window falls for your dates.
Why the date can change
- Irregular cycles or an uncertain period date.
- An early scan showing the baby is a little bigger or smaller than expected for your dates.
- Twins and some medical conditions, which often lead to earlier delivery.
Treat the due date as the middle of a window, not an appointment. To count down to it, use days until; after the birth, track your baby’s age in weeks with the baby age calculator.
This guide gives general information, not medical advice. Your doctor or midwife will confirm your dates.