Pregnancy Due Date Calculator
Enter the first day of your last period and your average cycle length to get your estimated due date, current gestational age, and trimester.
How Pregnancy Due Date Calculator works
Naegele's rule
The most widely used method for estimating a due date is Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This is then adjusted for cycle length — if your cycle is longer or shorter than 28 days, the estimated ovulation date shifts accordingly. The calculator applies this adjustment automatically by adding (cycle_length − 28) days.
Gestational age vs fetal age
Gestational age is measured from the first day of the last period, not from conception. Because conception typically occurs around 14 days after the LMP (for a 28-day cycle), gestational age is approximately 2 weeks more than fetal age. The 40-week pregnancy duration measured from LMP corresponds to roughly 38 weeks from conception.
Dating scan accuracy
An ultrasound dating scan in the first trimester (ideally between 10 and 13+6 weeks) is more accurate than LMP-based calculation for determining gestational age. The NHS offers a dating scan as part of standard antenatal care. If the scan date differs from the LMP-calculated date by more than 5–7 days, the scan date is generally used to recalculate the due date.
Due date reliability
Only around 4–5% of babies arrive exactly on their due date. About 70% arrive within 10 days before or after the due date. The NHS considers 37–42 weeks as full term. Babies born before 37 weeks are preterm; after 42 weeks is post-term. If labour has not started naturally by 41–42 weeks, induction is typically offered.
Frequently asked questions
How is my due date calculated?
Your due date is estimated by adding 280 days to the first day of your last menstrual period, adjusted for your cycle length. If your cycle is longer than 28 days, the due date is moved forward; if shorter, it moves back. An ultrasound scan in the first trimester gives a more precise date.
Can my due date change after a scan?
Yes — if your first-trimester dating scan shows a different gestational age from what the LMP calculation suggests, the NHS typically adjusts the due date to match the scan. Scan dates are more reliable in early pregnancy because fetal growth at this stage is less variable.
What is the difference between gestational age and fetal age?
Gestational age counts from the first day of your last period, while fetal age (or embryonic age) counts from conception. Conception typically occurs about 2 weeks after the LMP, so gestational age is about 2 weeks more than fetal age. Doctors always use gestational age.
When do I need to worry about going overdue?
The NHS considers 40 weeks as the due date but 37–42 weeks as full term. If your pregnancy reaches 41 weeks, your midwife or obstetrician will likely discuss induction options. At 42 weeks, induction is routinely offered because the risk of complications increases. Going a few days past your due date is very common and not cause for immediate concern.
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This calculator provides estimates for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.