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.
Reviewed by Richard Ross · Last updated April 2026
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.
How is my due date calculated?
The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. For cycles shorter or longer than 28 days, the due date is adjusted accordingly. An early ultrasound scan (8-12 weeks) measures the foetus and gives a more accurate estimate based on actual size rather than LMP. If there is a significant discrepancy, the scan date takes precedence.
What is the difference between gestational age and foetal age?
Gestational age is counted from the first day of the last menstrual period — so pregnancy is technically "2 weeks old" at the point of conception. Foetal (or embryonic) age counts from actual fertilisation and is typically 2 weeks less than gestational age. Medical professionals always use gestational age, so a "6-week scan" shows an embryo that is actually about 4 weeks old from fertilisation. This causes confusion — your midwife and your anatomy may appear to disagree.
How accurate is the estimated due date?
Only about 4% of babies are born on their estimated due date. Around 80% of births occur between 37 and 42 weeks. An early ultrasound (before 14 weeks) can place your due date within ±5-7 days. A later scan is less accurate. Many factors affect timing: first-time mothers tend to deliver later (average 41+2 for first babies vs 40+5 for subsequent pregnancies); some ethnic groups have different average gestation lengths; and post-term (after 42 weeks) risk of complications increases, prompting most NHS trusts to offer induction by 41+3.
What is a full-term pregnancy?
The American College of Obstetricians and Gynecologists (ACOG) defines full term as 39+0 to 40+6 weeks. Early term is 37+0 to 38+6 weeks; late term is 41+0 to 41+6 weeks; post-term is 42+ weeks. Most NHS maternity units aim to avoid deliveries before 39 weeks unless medically necessary, as outcomes are consistently better from 39 weeks. Babies born at 37-38 weeks (early term) have higher rates of breathing problems, feeding difficulties, and NICU admission than those born at 39+ weeks.
What happens if I go past my due date?
Going past 40 weeks is common — about 50% of pregnancies continue beyond the due date. NHS guidelines recommend a membrane sweep from 40 weeks to encourage natural labour. If pregnancy continues to 41 weeks, most NHS trusts will offer induction. After 42 weeks, the risk of stillbirth increases and induction or monitoring is strongly recommended. Induction at 41+3 has been shown to reduce perinatal mortality without increasing C-section rates (the "ARRIVE" and "SWEPIS" trial findings).
Does IVF change how the due date is calculated?
Yes. For IVF pregnancies, the due date is calculated from the egg retrieval date (or embryo transfer date, minus 2 weeks for fresh cycles or minus 5 days for day-5 blastocyst transfers). This is more precise than LMP-based calculation for natural conception because the date of fertilisation is known exactly. IVF pregnancies tend to be monitored more closely and have slightly different risk profiles, but the gestational milestones and due date calculation method is otherwise the same.
Related calculators
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.