MayoCalc / Health

Due Date Calculator

Estimate your baby's due date and see how far along you are.

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Estimated Due Date
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Disclaimer: This tool is for general educational and informational purposes only. It does not constitute medical advice, professional diagnosis, or treatment and is not a substitute for consultation with a qualified healthcare provider. No doctor-patient relationship is created by your use of this tool. Results are estimates based on published formulas and population-level data that may not reflect your individual medical circumstances. Neither MayoCalc nor Cook Media Systems assumes any liability or responsibility for damage or injury (including death) to any person arising from the use of any information, results, or content provided by this tool. Always consult your physician or other qualified healthcare provider with any questions about a medical condition. See our full Disclaimer and Terms of Service.

How Due Dates Are Calculated

The standard method for calculating a pregnancy due date is Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day menstrual cycle with ovulation occurring on day 14. If your cycle is longer or shorter, the due date shifts accordingly. Only about 5% of babies are born on their exact due date. Most full-term births occur within a window of 37-42 weeks.

Due Date = First Day of LMP + 280 days
Or: Due Date = Conception Date + 266 days

How to Use This Calculator

Enter the first day of your last menstrual period and your average cycle length. The calculator shows your estimated due date, conception date, the end of each trimester, and a week-by-week timeline of your pregnancy. If you know your conception date, you can enter that instead for a more precise estimate.

Trimester Timeline

First trimester (weeks 1-12): Organ development begins. Most pregnancy symptoms (nausea, fatigue) occur during this period. Second trimester (weeks 13-26): Often called the "golden trimester" because symptoms typically ease. The anatomy scan usually occurs at weeks 18-22. Third trimester (weeks 27-40): Rapid fetal growth. The baby gains most of its weight during this period. The Pregnancy Weight Calculator shows recommended weight gain by trimester.

Due Date FAQ

How accurate is the due date?
A due date based on LMP is an estimate with a margin of about 2 weeks. First-trimester ultrasound dating (before 13 weeks) is more accurate, typically within 5-7 days. If the ultrasound date differs from the LMP date by more than 7 days, most providers will adjust the due date to match the ultrasound.
What if my cycle is not 28 days?
If your cycle is longer (say 35 days), you likely ovulated later, so your due date is later than the standard calculation suggests. This calculator adjusts for cycle length. Enter your actual average cycle length for a more accurate estimate. Irregular cycles make LMP-based dating less reliable; ultrasound dating is preferable in these cases.

How Due Dates Are Calculated

The standard due date calculation uses Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14, which is an approximation since cycle lengths vary. For cycles longer or shorter than 28 days, the due date can be adjusted by adding or subtracting the difference. First-trimester ultrasound measurements (crown-rump length) are the most accurate method of confirming gestational age and can adjust the EDD if it differs from the LMP calculation by more than 5-7 days.

Only about 5% of babies are born on their exact due date. Full-term pregnancy spans 37 to 42 weeks, with most births occurring between 39 and 41 weeks. The March of Dimes considers 39-40 weeks to be "full term," 41 weeks "late term," and 42+ weeks "post-term." First-time mothers tend to deliver slightly later (averaging 40 weeks and 5 days) than mothers who have given birth before. Labor induction is typically discussed between 39 and 41 weeks depending on maternal and fetal health factors.

Pregnancy Milestones by Trimester

The first trimester (weeks 1-12) is the period of greatest organ development and highest miscarriage risk. The second trimester (weeks 13-27) is often considered the most comfortable, with reduced nausea and increased energy. The third trimester (weeks 28-40) brings rapid fetal growth and preparation for birth. Key milestones include: heartbeat detectable at 6-7 weeks, anatomy scan at 18-22 weeks, viability threshold at approximately 24 weeks, and fetal lung maturity typically by 36-37 weeks. The Pregnancy Weight Calculator tracks recommended weight gain throughout each trimester.

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