Pregnancy due date calculator
Due date and how many weeks pregnant
For regular cycles of 21 to 35 days. A due date set by ultrasound takes priority over this calendar estimate. Your dates are not sent anywhere.
- Weeks pregnant
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From 37+0 to 42+0 weeks
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Worked out in your browser. No sign-up.
The due date calculator counts 280 days, or 40 weeks, from the first day of your last period, then adjusts for a cycle longer or shorter than 28 days. Pick any date to see how far along you are in weeks and days. It is a calendar estimate; the due date from your dating scan always takes priority.
How it works
For a 28-day cycle the due date is the first day of your last period plus 280 days. If your cycle is longer, ovulation usually comes later, so the calculator adds the difference: last period + 280 + (cycle length − 28) days. A 31-day cycle moves the date three days on.
Weeks pregnant are counted the same way, from the last period rather than from conception, with the same cycle correction. The result is shown as full weeks plus days, the way midwives write it (6+3). The 37 to 42 week range is shown for reference only. It isn't advice on when to book anything; your midwife or doctor decides that.
Worked example
Anna's last period started on 1 January 2026 and her cycle is a steady 28 days. Her estimated due date is 8 October 2026. On 12 February she is 6 weeks and 0 days pregnant, written 6+0.
If her cycle were 30 days long, the due date would move to 10 October.
Questions people ask
Why are pregnancy weeks counted from before I conceived?
Because the first day of your last period is a date most people know, and conception usually isn't. With a 28-day cycle that adds about two weeks, so at the moment of conception you are already counted as 2 weeks pregnant.
What does 6+3 mean on my notes?
Six full weeks and three days. In everyday speech that is often called being in your seventh week.
Does it work if my periods are irregular?
Not reliably. There's no single cycle length that corrects for ovulation moving around from month to month. An early ultrasound scan is the usual way to date the pregnancy in that case.
How many babies arrive on their due date?
Only a small share. The due date is the middle of a window, not an appointment, and most babies are born somewhere between 37 and 42 weeks.
Sources
- NHS: Due date calculator (last period, cycle length, dating scan) — checked 13.11.2026
- ACOG: Methods for Estimating the Due Date (280 days, 28-day cycle) — checked 13.11.2026
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Updated: 13 November 2026