Pregnancy Due Date Calculator
Estimate your due date and current pregnancy week from the last menstrual period and cycle length.
Written by Rahman · Last reviewed
Quick answer
A due date is conventionally the first day of the last menstrual period plus 280 days, adjusted for cycle length. Pregnancy is dated from the LMP rather than from conception, which happens roughly two weeks later, so the first two weeks of a pregnancy precede it. Fewer than 5% of babies arrive on the estimated date.
Based on Naegele's rule (LMP + 280 days), adjusted for cycle length. This is an estimate — only about 4% of babies arrive on their due date, and an early ultrasound dates a pregnancy more accurately than the last period does.
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About the Pregnancy Due Date Calculator
Pregnancy is conventionally measured in weeks from the first day of your last menstrual period (LMP), not from conception, which usually occurs about two weeks after the LMP. A typical pregnancy lasts 40 weeks (280 days) from LMP.
This calculator uses Naegele's rule to estimate your due date, adjusting for cycle lengths that differ from the typical 28 days. It also shows your current pregnancy week, the end of each trimester and the approximate date of conception.
Due date = LMP + 280 days (40 weeks), adjusted by (cycle length − 28) days.
Most accurate when LMP is known and cycles are regular. Ultrasound dating is more accurate for irregular cycles or when the LMP date is uncertain.
Why the due date is an estimate, not an appointment
Naegele's rule adds 280 days to the first day of the last menstrual period. It assumes a 28-day cycle with ovulation on day 14, which is why this calculator adjusts for cycle length.
- Fewer than 5% of babies are born on their estimated due date.
- Most births occur within roughly two weeks either side of it.
- The rule depends on knowing the LMP accurately and on regular cycles; irregular cycles make it substantially less reliable.
- A first-trimester ultrasound dates a pregnancy more accurately than LMP and is what clinicians use when the two disagree.
Treat the date as the midpoint of a range rather than a deadline. Antenatal care, not a calculator, is what should drive any decision; this tool exists to tell you roughly where you are in the timeline.
Frequently Asked Questions
How accurate is the due date?
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Fewer than 5% of babies are born on their estimated due date. Most births happen within two weeks before or after. The estimate is a guide, not a prediction: in a study that dated conception precisely by hormone measurements, the length of pregnancy still spanned 37 days once preterm births were excluded.
What if my cycle is longer or shorter than 28 days?
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Adjust the cycle length input. The calculator shifts the due date by the difference (e.g., a 30-day cycle pushes the date back two days; a 26-day cycle pulls it forward two days).
When is the first trimester?
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Weeks 1–13 are the first trimester, 14–27 the second, and 28 to birth the third. The calculator shows the exact end dates of trimesters one and two.
Should I rely on this if my LMP is uncertain?
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No. Ask your doctor for an ultrasound-based dating scan, which is far more accurate than LMP-based dating for women with irregular cycles or uncertain dates.
Sources
- ACOG — Methods for Estimating the Due Date (Committee Opinion 700) — US obstetric guidance on how the estimated due date is set and revised. Backs the point that ultrasound outranks LMP: it calls a pregnancy without an ultrasound confirming or revising the date before 22 0/7 weeks "suboptimally dated".
- Jukic et al. (2013), Human Reproduction — Length of human pregnancy and contributors to its natural variation — 125 naturally conceived pregnancies with ovulation pinned by urinary hormones. Median ovulation-to-birth was 268 days, and the spread was 37 days even after excluding preterm births, which is the evidence that a due date is an estimate. Also found dating by LMP more variable than dating by ovulation.