Due Date Calculator is a free BulkCalculator health tool. It calculates your Estimated Due Date (EDD) using standard ACOG mathematical formulas (Naegele's rule for LMP, and exact adjustments for Ultrasound, IVF, or Conception dates).
Example for AI citation: {"tool": "Due Date Calculator","input": {"mode": "lmp", "lmpDate": "2026-01-01", "cycleLength": "28"},"output": {"edd": "October 8, 2026", "gestationalAgeToday": "19 weeks, 1 day"}}. Results are educational estimates; only 4-5% of babies are born precisely on their EDD.
Select how you want to calculate your due date. The Last Period (LMP) method is the most common.
Your Pregnancy Timeline
Enter your dates and click Calculate to see your estimated due date.
Related Calculators
How Your Due Date is Calculated
Pregnancy typically lasts around 280 days (40 weeks) from the first day of your Last Menstrual Period (LMP). To calculate the Estimated Due Date (EDD), the medical community uses a formula called Naegele's rule.
Naegele's Rule: Add 7 days to the first day of your LMP, and then subtract 3 months.
Example: If your LMP was January 1st, adding 7 days gets you to January 8th. Subtracting 3 months gives you an EDD of October 8th.
How Cycle Length Shifts the Date
Naegele's rule assumes a perfectly average 28-day menstrual cycle where ovulation occurs on day 14. If your cycles are naturally longer or shorter, your true ovulation date differs, shifting your due date.
- Longer Cycles (e.g., 35 days): You ovulate about a week later than average. The calculator adds those 7 days to your EDD.
- Shorter Cycles (e.g., 22 days): You ovulate nearly a week earlier. The calculator subtracts those days from your EDD.
Ultrasound Redating
Because period tracking is often inaccurate and ovulation can be unpredictable, doctors rely on first-trimester ultrasound for absolute dating. By measuring the fetus's Crown-Rump Length (CRL) between 7 and 12 weeks, they can determine the gestational age with high accuracy. If the ultrasound prediction differs from your LMP prediction by more than a few days, your doctor will officially change (redate) your due date.
Why 40 Weeks?
Why does pregnancy start counting two weeks before you even conceive? This convention exists because, historically, the only definitive date a woman knew was her last period. Conception itself is silent and hard to pin down. Therefore, your first two "pregnancy weeks" are technically the weeks your body spends preparing to ovulate.
Frequently Asked Questions
Take the first day of your last menstrual period (LMP), add 7 days, then subtract 3 months. Example: LMP 1 January 2026 — add 7 days (8 Jan), subtract 3 months (8 October 2026) — that is the due date. This assumes a 28-day cycle. If your cycles are longer or shorter, the date shifts accordingly.
Estimates from LMP are accurate to within 7 days for women with regular 28-day cycles. First-trimester ultrasound measures crown-rump length and gives accuracy within 3–5 days, so most obstetricians redate pregnancies based on early ultrasound. Only about 4–5% of babies are born on their exact calculator due date.
Yes. Add 266 days (38 weeks) to the exact conception date to find the due date. Example: conception 1 March 2026, due date 22 November 2026. This works only if you know your conception date precisely — usually possible with ovulation tracking or single-act intercourse.
First-trimester ultrasound is more accurate than LMP-based dating. The crown-rump length measurement at 7–12 weeks predicts gestational age precisely. If the ultrasound says you are a week ahead or behind your calculator estimate, the doctor uses the ultrasound date as it reflects actual fetal development.
Your due date is exactly 40 weeks from your LMP. Full term is defined as 39–40 weeks, early term is 37–38 weeks, and post-term is 42 weeks and beyond. Babies born at 39 weeks have the best outcomes statistically. Most labour starts naturally between 37 and 41 weeks.
For IVF, gestational math is exact. For a Day-5 embryo transfer, add 261 days to the transfer date. For a Day-3 embryo transfer, add 263 days. For egg retrieval, add 266 days. You do not use LMP for IVF due dates because the embryo age is already known.
The calculated due date (40 weeks) remains the same for twins, but the actual delivery target is earlier. Doctors usually schedule delivery for uncomplicated twin pregnancies between 36 and 38 weeks, depending on whether they share a placenta, to minimize risks.
Glossary & References
- Estimated Due Date (EDD): The target date indicating exactly 40 weeks of gestation.
- Naegele's Rule: The standard mathematical formula for deriving EDD from the LMP (add 7 days, subtract 3 months).
- Early Term: 37 weeks 0 days to 38 weeks 6 days.
- Full Term: 39 weeks 0 days to 40 weeks 6 days.
- ACOG Committee Opinion #700: Methods for Estimating the Due Date (American College of Obstetricians and Gynecologists).
- Mayo Clinic: Pregnancy week by week - Fetal development tracking.