Due Date Calculator
Estimates your baby's due date by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP) using Naegele's Rule. Use it in early pregnancy to plan prenatal appointments, milestones, and parental leave.
Last updated: May 2026
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About this calculator
The most widely used method for estimating a due date is Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). The rule assumes a regular 28-day menstrual cycle with ovulation on day 14 of the cycle. The formula is: Estimated Due Date (EDD) = LMP + 280 days. Days remaining until delivery = EDD − Today's Date. Variables: LMP (the first day of the last menstrual period). Because 280 days spans roughly 9 calendar months and 7 days, the result aligns with the standard 40-week gestational calendar. Only about 5% of babies are born on their exact EDD; the actual delivery typically falls within a two-week window (37–42 weeks). Edge cases: women with cycles substantially shorter or longer than 28 days need a corrected EDD — for a 32-day cycle, add 4 days (the extra time before ovulation) to the standard estimate. Recent hormonal contraceptive use, breastfeeding, irregular periods, or perimenopause make the LMP unreliable. A first-trimester crown-rump-length ultrasound (8–13 weeks) is the gold-standard dating method and is accurate to ±5–7 days; current ACOG/RCOG guidelines recommend that ultrasound-derived dates override LMP dates when they differ by more than 7 days in the first trimester. IVF pregnancies use the embryo transfer date for precise dating.
How to use
Example 1: Your LMP began on March 1, 2025. Step 1: add 280 days to March 1, 2025 → December 5, 2025 (estimated due date). Step 2: if today is May 15, 2025, days remaining = December 5 − May 15 = 204 days. Verify: 40 weeks × 7 = 280 days; counting forward from March 1 lands on December 5 — confirmed. Example 2: Your LMP was 70 days ago. Step 1: days remaining = 280 − 70 = 210 days. Step 2: convert to weeks pregnant — 70 / 7 = 10 weeks gestational age. Verify: at 10 weeks you are early second-month, with roughly 30 weeks remaining (210 / 7 = 30) — consistent with a 40-week pregnancy.
Frequently asked questions
How accurate is the due date calculator based on last menstrual period?
The LMP-based due date is accurate to within about ±2 weeks for most pregnancies with regular 28-day cycles. It can be less precise if your cycles are irregular, longer or shorter than 28 days, or if you recently stopped hormonal contraception. A first-trimester ultrasound (ideally between 8 and 13 weeks) is the gold-standard confirmation and is accurate to within 5–7 days. Most healthcare providers use both methods together, with ultrasound dating taking precedence when discrepancies exceed 7 days in early pregnancy. Only about 5% of babies are born on their exact estimated due date — most arrive between 37 and 42 weeks of gestation.
What does 280 days mean in pregnancy and why is it used?
280 days equals 40 weeks, the internationally accepted average length of a human pregnancy measured from the first day of the last menstrual period. Clinicians count from the LMP rather than from conception because the exact day of ovulation and fertilization is rarely known. This 40-week framework divides neatly into three trimesters of roughly 13 weeks each and aligns with the developmental milestones tracked at prenatal visits. Even though conception typically occurs around day 14 of the cycle, adding 14 days to the LMP start brings the same total — fetal age (post-conception) is therefore about 38 weeks, two weeks less than gestational age. The 280-day figure originates from German obstetrician Franz Naegele's observations in the early 1800s and remains the global standard.
When should I use a due date calculator instead of asking my doctor?
A due date calculator is a great first step as soon as you suspect you are pregnant — it gives an immediate estimate before your first prenatal appointment. It helps you plan when to book your first ultrasound, schedule time off work, and begin prenatal vitamins. However, your doctor or midwife will confirm or adjust the date using ultrasound measurements (crown-rump length in the first trimester is the most reliable) and clinical assessment. Use this tool for planning and orientation rather than as a medical diagnosis, and update your records once a clinical estimate is available. For IVF pregnancies, the clinic's transfer-date-based EDD supersedes the LMP method entirely.
What are common mistakes when calculating a due date from the LMP?
Using the date your period ended rather than the first day it began shifts the EDD by 3–7 days. Forgetting that 'pregnant' is conventionally counted from the LMP and not from conception leads to confusion when discussing weeks pregnant versus fetal age. Applying Naegele's Rule to a woman with a 35-day cycle without adjustment makes the EDD about a week too early. Not accounting for recent hormonal contraceptive use, breastfeeding, or PCOS — all of which can disrupt cycle regularity — produces unreliable estimates. Confusing the EDD with a guarantee rather than a midpoint of a delivery window (37–42 weeks term, with only 5% of babies born exactly on the EDD) leads to unrealistic planning. Finally, using LMP dating when first-trimester ultrasound is available wastes a more accurate measurement.
When should I NOT use an LMP-based due date calculator?
Do not rely on LMP dating if you have irregular cycles, recently stopped hormonal contraception (within 3 months), are breastfeeding, or have polycystic ovary syndrome (PCOS) — in these cases, ovulation timing is uncertain and the standard formula can be off by weeks. IVF and other assisted-reproduction pregnancies should use the embryo transfer date for precise dating, not LMP. If you don't remember your LMP date, only a first-trimester ultrasound (crown-rump length) can establish accurate gestational age. After 22 weeks, dating becomes progressively less reliable from any method, so dates established later in pregnancy carry larger error margins. Finally, for clinical decisions — preterm labor management, scheduled cesarean section timing, induction planning — always defer to your obstetric provider's records rather than an online estimate.