EDD Calculator - Estimated Date of Delivery

About estimated delivery dates

An estimated date of delivery, or EDD, provides a standard reference point for pregnancy timing and prenatal planning. Pregnancy is conventionally dated as 40 weeks, or 280 days, from the first day of the last menstrual period. This convention begins about two weeks before conception in a typical 28-day cycle. When a reliable conception or ovulation date is known, the estimate is 266 days from that date. Neither method predicts the exact day labor will begin; they establish a useful clinical timeline. The last-menstrual-period method applies Naegele's rule and adjusts for average cycle length. A cycle longer than 28 days moves the estimate later by the difference, while a shorter cycle moves it earlier. This adjustment assumes ovulation changes with cycle length and that the reported period was a true, accurately remembered menstrual period. Irregular cycles, recent hormonal contraception, breastfeeding, bleeding in early pregnancy, and uncertain dates can make an LMP estimate less reliable. Ultrasound dating uses the gestational age assigned on the scan date. The calculator subtracts that age from 40 weeks and adds the remaining days to the scan date. Early first-trimester ultrasound is generally more accurate for dating than later measurements because normal variation in fetal size increases as pregnancy progresses. Clinicians follow established guidelines when deciding whether an ultrasound difference is large enough to revise an existing due date. Assisted-reproduction pregnancies are dated from known retrieval, fertilization, or embryo-transfer information using protocol-specific conventions. Only a small proportion of births occur on the estimated date itself. A healthy term window extends around it, and individual management depends on maternal health, fetal growth, previous pregnancy history, test results, and local guidance. Keep the date recorded by the prenatal care team as the authoritative EDD, especially if it differs from a simple calculator. Contact a maternity professional about bleeding, fluid leakage, painful contractions, reduced fetal movement, severe headache, or other urgent concerns. This calculator supports general planning and education and cannot confirm pregnancy, fetal health, or the timing of labor.

EDD calculation examples

How to estimate your delivery date

1. Choose LMP, known conception date, or ultrasound dating.
2. Enter the date and any cycle-length or gestational-age details.
3. Select Calculate Due Date to apply the appropriate clinical convention.
4. Discuss the estimate and any conflicting dates with your prenatal care team.

Frequently asked questions