Due Date Calculator

Work out a baby due date two ways: from the first day of your last period (adding 280 days) or from a known conception date (adding 266 days).

How it works: From a last period, the due date is 280 days (40 weeks) later; from a known conception date it is 266 days (38 weeks) later. Only about 1 in 20 babies arrive exactly on the due date — it is a planning estimate, not a promise.

How the Due Date Calculator Works

This calculator estimates a baby’s due date from the first day of the last menstrual period (LMP) using the standard Naegele’s rule: add 280 days (40 weeks) to the LMP. The rule assumes a regular 28-day cycle with ovulation on day 14, and it counts pregnancy from the LMP rather than from conception, because the LMP is usually known while the conception day often is not. The result is the estimated date of delivery (EDD) — the day the pregnancy reaches exactly 40 weeks.

The arithmetic can be done two equivalent ways. Adding 280 days directly gives the same answer as Naegele’s shortcut: add one year, subtract three months, add seven days. For an LMP of 10 February 2026, adding 280 days lands on 17 November 2026; the shortcut gives the same date (10 February + 1 year = 10 February 2027, − 3 months = 10 November 2026, + 7 days = 17 November 2026). The calculator also shows the current gestational age in weeks and days when you supply today’s date, and the trimester (weeks 1–13 first, 14–27 second, 28–40 third).

If you know the conception date instead, the equivalent rule is conception + 266 days (38 weeks), because conception happens about 14 days after the LMP. A conception date of 24 February 2026 therefore gives the same 17 November 2026 due date. The calculator also places key milestones on the timeline: first heartbeat scan around 6–7 weeks, anatomy scan around 18–20 weeks, viability around 24 weeks.

How to Use This Calculator

Enter the first day of your last menstrual period.
Press Calculate to see the estimated due date.
Note the gestational age and trimester shown alongside, if you are currently pregnant.
Use the milestone dates to plan appointments — they are guides, not medical advice.
If your cycles are irregular, treat the result as approximate and confirm with an early ultrasound dating scan.

Example

If the first day of the last period was 10 February 2026, the due date is 17 November 2026 (10 February + 280 days). On 10 August 2026, the pregnancy is 26 weeks and 0 days (26w0d) — in the second trimester, two weeks short of the third.

If conception happened on 24 February 2026, the due date is again 17 November 2026 (24 February + 266 days), because 24 February is 14 days after the 10 February LMP. The two methods agree whenever the cycle is regular.

Understanding Your Result

The due date is an estimate, not a deadline. Only about 5% of babies arrive on their due date; most arrive within two weeks either side, and the range considered full term runs from 39 weeks to 40 weeks and 6 days. First pregnancies often run a few days past the due date. Clinicians use the date to schedule care — screenings, scans and check-ups are organised by gestational week — so treat it as the anchor for planning, not as a prediction of the birth day.

The 280-day rule assumes a regular 28-day cycle. Longer cycles push ovulation later and the true due date later; shorter cycles pull it earlier. A dating scan at 8–12 weeks measures the embryo and is more accurate than the calendar method, so it often adjusts the due date by a few days. If the scan date and the calculator date differ by more than a week, clinicians usually adopt the scan date.

The week-and-day format (for example, 26w0d) is how pregnancy care is organised, and small differences matter: a screening test offered at 11–14 weeks or an anatomy scan at 18–20 weeks must fall inside its window, so a date a week out can move an appointment. The trimester boundaries used here — second trimester from week 14, third from week 28 — are the common convention, though some clinicians use slightly different cut-offs; confirm with your care team if a result sits near a boundary.

Due Date Calculator FAQs

Why is the due date 40 weeks from the last period?

Because pregnancy is conventionally counted from the first day of the last period, which is usually known, even though conception happens about two weeks later. The baby develops for about 38 weeks, but the count runs 40 weeks from the LMP.

How accurate is the due date?

It is an estimate based on a regular 28-day cycle. An early ultrasound dating scan is more accurate and often adjusts the date by a few days; only about 5% of babies are born exactly on their due date.

What if my cycles are irregular?

The 280-day rule assumes ovulation on day 14. With irregular cycles, ovulation may fall earlier or later, shifting the true due date. Use the result as a guide and confirm with a scan.

What does 26w0d mean?

It means 26 weeks and 0 days of pregnancy — 182 days since the first day of the last period. The next day becomes 26w1d.

Can the due date change later in pregnancy?

Yes — a dating scan can revise it, usually by a few days, and clinicians adopt the scan date when the difference is significant. Later scans are less reliable for dating because babies grow at different rates.