Free Pregnancy Due Date Calculator
Calculate your pregnancy due date using Naegele's Rule — the standard obstetric method used worldwide. Enter your last menstrual period date (or conception date) to get your estimated due date, current gestational age, trimester, and key milestone dates. Free, private — all calculations run in your browser.
This calculator provides an estimated due date for informational purposes only. Consult your OB/GYN or midwife for a confirmed due date based on clinical examination and ultrasound.
About This Due Date Calculator
This pregnancy due date calculator uses Naegele's Rule — the standard obstetric formula used by obstetricians and midwives worldwide to estimate the Estimated Due Date (EDD). Named after German obstetrician Franz Naegele (1778–1851), the rule states that a pregnancy lasts 280 days (40 weeks) from the first day of the last menstrual period. It is the basis for due date calculation in most electronic health record systems and obstetric apps globally.
The Formula — How Due Dates Are Calculated
The standard Naegele's Rule calculation:
Gestational Age = (Today − LMP) ÷ 7 [weeks]
Equivalently: subtract 3 months from the LMP date, then add 7 days and 1 year. For cycles other than 28 days, the calculator adjusts the due date by the difference between your cycle length and 28 days (e.g., a 35-day cycle pushes the due date 7 days later).
Why Only 5% of Babies Are Born on Their Due Date
The 280-day estimate is a population average, not a biological certainty. The duration of human pregnancy varies naturally by several weeks. Studies show that among uncomplicated pregnancies, the actual range of natural labour onset spans approximately 37 to 42 weeks, with the median for first-time mothers approximately 5 days after the EDD. A due “date” is more accurately thought of as a due “fortnight” — the midpoint of a normal 4-week window.
Alternative Due Date Methods
Several clinical methods can confirm or supersede the LMP-based due date:
- •First-trimester ultrasound (6–10 weeks): Crown-rump length measurement is the most accurate method, reliable to ±5 days.
- •Second-trimester ultrasound (14–20 weeks): Accurate to ±10–14 days — less precise than first-trimester scanning.
- •IVF transfer date: When the conception date is known exactly, due date calculation is highly precise.
- •Known ovulation date: If tracked via basal body temperature or ovulation test, can be used instead of assuming Day 14.
The Three Trimesters
Pregnancy is divided into three trimesters of roughly 13 weeks each. The first trimester (weeks 1–12) is the period of highest miscarriage risk and most rapid embryonic development. The second trimester (weeks 13–27) is typically the most comfortable — morning sickness usually resolves, energy returns, and the baby's movements become detectable. The third trimester (weeks 28–40) is the final growth phase, during which the baby gains most of its birth weight and the lungs mature in preparation for delivery.
Privacy Notice
All calculations run entirely in your browser. No data you enter — including your LMP date — is transmitted to any server or stored anywhere. Your information stays completely private on your device.
When to Use This Calculator
Calculate your due date to determine the optimal start date for maternity leave, typically 11 weeks before in the UK or negotiated based on your due date.
Baby showers are traditionally held 4–6 weeks before the due date. Calculate your due date to pick a date that falls in the comfortable third trimester window.
Childcare waiting lists can be 12–18 months long. Knowing your due date early helps you get on the right waitlists before your baby arrives.
Airlines restrict travel from 36 weeks for single pregnancies. Calculate your due date to identify the window when international travel is still possible.
Plan your birth announcement timing, photographer bookings, and social media posts — knowing your approximate due date window helps you prepare in advance.
💡 Pro Tips
An early ultrasound scan at 6–10 weeks is more accurate than LMP-based calculation for dating your pregnancy. The fetal crown-rump length measurement at this stage can pinpoint gestational age to within ±5 days. If the ultrasound date differs from your LMP date by more than 7 days, your care provider will typically revise the estimated due date to match the ultrasound measurement.
Your due date is a midpoint, not a deadline. Normal term pregnancy spans 37–42 weeks. About 80% of babies arrive between 38 and 41 weeks — only 4–5% land on the actual due date. Plan your preparations to be ready 2 weeks before your due date, and try not to feel overdue until after 41 weeks.
Start tracking your baby's movements from approximately 28 weeks (third trimester). You should feel 10 movements within a 2-hour window. Reduced or absent movement is the most important warning sign to monitor in late pregnancy. Apps like "Count the Kicks" can help you track patterns. If in doubt, contact your maternity unit immediately — they would rather check and reassure you than have you wait.
Pack your hospital bag by 36 weeks, not 40 weeks. First-time mothers statistically go into labour a few days after their due date on average, but up to 10% of first-time mothers go into labour before 37 weeks. Having your bag ready at 36 weeks means you are always prepared, with a safety margin of 4 weeks before your due date.
Frequently Asked Questions
Medical Disclaimer
Due Date Calculator — Results are estimates based on Naegele's Rule and are intended for informational and planning purposes only. This calculator does not constitute medical advice. Your official Estimated Due Date (EDD) should be confirmed by a qualified OB-GYN, midwife, or other licensed obstetric care provider using clinical assessment, including early ultrasound. Do not make medical or clinical decisions based on this calculator alone. If you have any concerns about your pregnancy, contact your healthcare provider immediately.
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