Pregnancy due date calculator

How to use the calculator

1. Choose whether to calculate from your last period or from a known conception date.
2. Enter the first day of your last period — the day bleeding started — or the conception date.
3. If your cycle is usually shorter or longer than 28 days, enter its usual length. This moves the due date by the difference.
4. Read the estimated due date, how far along you are today, and the dates of the dating-scan window and the full-term window.

How the due date is estimated

Pregnancy is counted from the first day of the last menstrual period, not from conception, because that is the date most people know. A typical pregnancy lasts, on average, 280 days or 40 weeks counted this way, as the University of Rochester Medical Center explains; the due date is therefore the first day of the last period plus 280 days. That assumes a 28-day cycle with ovulation around day 14. If your cycle is usually 35 days, ovulation — and the due date — are about a week later; the NHS due date calculator likewise asks for cycle length for this reason. When the conception date is known, the due date is that date plus 266 days, which is the same thing counted from two weeks later.

Naegele's rule

The traditional shortcut, Naegele's rule, takes the first day of the last period, counts back three calendar months and adds one year and seven days. Because calendar months have different lengths, it does not always match 280 days exactly. For a last period starting on 1 March 2026, 280 days gives 6 December 2026, while Naegele's rule gives 8 December. The calculator shows both when you use a 28-day cycle.

Why the scan matters

Dates based on the last period depend on remembering it correctly and on a regular cycle. The American College of Obstetricians and Gynecologists states in Committee Opinion No. 700 that the due date should be set as soon as the last period, the first accurate ultrasound, or both are known, and that a pregnancy without an ultrasound confirming or revising the due date before 22 weeks should be considered suboptimally dated. In the UK, the NHS offers a dating scan at around 10 to 14 weeks. Once your care team has set a due date, use theirs.

Full term

Very few babies arrive on the due date itself. The NHS describes pregnancy as normally lasting from 37 to 42 weeks from the first day of the last period, and the calculator shows that window as dates.

Examples

With a last period on 1 March 2026, on 28 September 2026 the pregnancy is 211 days along — 30 weeks and 1 day — with 69 days to the due date. The dating-scan window runs from 10 May to 7 June 2026 and the full-term window from 15 November to 20 December 2026.

Limits

• An estimate, not a diagnosis. It cannot confirm a pregnancy or say anything about its health.
• Irregular cycles make the last-period method unreliable. If your cycle varies a lot, the dating scan is the better guide.
• IVF and assisted conception are dated from the embryo transfer by the clinic; follow the date they give you.
• Twins and other multiple pregnancies often deliver earlier. Your care team will plan accordingly.

Sources and further reading

• NHS: Pregnancy due date calculator
• University of Rochester Medical Center: Calculating a due date
• ACOG Committee Opinion No. 700: Methods for Estimating the Due Date (Obstet Gynecol 2017)