Pregnancy Due Date Calculator

Your estimated due date is 40 weeks (280 days) after the first day of your last menstrual period, or the equivalent date worked out from conception, an IVF transfer or a dating scan. This calculator supports all of these methods, tells you how many weeks and days pregnant you are on any date, and lists the key milestones of the pregnancy.

How to use this tool

1. Choose what you know: your last period, conception date, IVF transfer or an ultrasound result.
2. Enter that date. For the LMP method, add your usual cycle length; for a scan, enter the weeks and days on the report.
3. Leave “Show progress on” as today, or pick another date to see how far along you will be then.
4. Read the due date and current week, then check the table of milestones.

Formula

Due date = LMP + 280 days + (cycle − 28); conception + 266; day-5 transfer + 261; day-3 transfer + 263; scan date + (280 − age at scan in days)

Gestational age is counted from the LMP-equivalent start date, which is 280 days before the due date. Trimesters: first until 13 weeks 6 days, second from 14 weeks to 27 weeks 6 days, third from 28 weeks. ACOG term definitions: early term 37w0d, full term 39w0d, late term 41w0d and post-term from 42w0d. Cycle adjustment assumes ovulation roughly 14 days before the next period; real cycles can differ. Use the due date confirmed by your clinician.

Example

With a last period starting on 1 January 2026 and a 28-day cycle, the due date is 1 January + 280 days = 8 October 2026 (Naegele’s rule gives the same date: add a year, subtract three months, add seven days). On 1 March 2026 that pregnancy is 59 days along, or 8 weeks 3 days, in the first trimester. A dating scan on 1 March measuring 8 weeks 3 days gives the same due date.