Work out your estimated due date from the first day of your last period. You’ll also see how many weeks pregnant you are, which trimester you’re in, and roughly when your 12-week and 20-week scans should be.
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How your due date is worked out
Doctors and midwives traditionally use Naegele’s rule: the due date is 280 days (40 weeks) after the first day of your last period. That assumes a 28-day cycle with ovulation around day 14. If your cycle is longer or shorter, the calculator moves the date by the difference.
Pregnancy is counted from your period, not from conception, so at conception you’re already considered about 2 weeks pregnant.
Worked examples
A 28-day cycle
If your last period started on 1 January 2026, your due date is 8 October 2026.
A 32-day cycle
With the same start date but a 32-day cycle, ovulation is likely about 4 days later, so the due date moves to 12 October 2026.
What happens next
- Dating scan (10 to 14 weeks): measures the baby and often adjusts your due date. The scan date is more accurate than the period date.
- Anomaly scan (18 to 21 weeks): checks the baby’s development in detail.
- Full term: babies born between 37 and 42 weeks are considered full term. Only about 1 in 20 arrive on the due date itself.
If you think you’re pregnant, contact your GP or a midwife, who will arrange your antenatal care. Start taking folic acid as early as you can.
Frequently asked questions
How accurate is a due date calculator?
It gives a good estimate, but only about 5% of babies are born on their due date. Your dating ultrasound scan gives a more accurate date.
How many weeks pregnant am I?
Count from the first day of your last period. The calculator shows weeks and days as of today.
What if I don't know the date of my last period?
Your midwife or doctor can date the pregnancy from an ultrasound scan instead.
Is this medical advice?
No. It’s an estimate. Always follow the dates and advice from your midwife or doctor.
Sources
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