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Estimates the expected date of delivery from the last menstrual period.
Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.
The estimated date of delivery (EDD) is a clinical estimate of the anticipated date of birth, calculated by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP), per the standard Naegele rule. This calculation assumes a regular 28-day menstrual cycle with ovulation occurring approximately 14 days before the next menses. Gestational dating is a cornerstone of prenatal care, guiding the scheduling of prenatal visits, screening tests (e.g., Down syndrome screening, glucose tolerance testing), surveillance of fetal growth, and timing of delivery. Accurate dating is essential for managing preterm labor, post-term pregnancies, and decisions about induction of labor. Important limitations: The EDD based on LMP is an estimate. It may be inaccurate if the patient has irregular cycles, uncertain recall of the LMP, or if ovulation occurred at a different point in the cycle. First-trimester ultrasound dating (crown-rump length measurement) is considered more reliable than LMP dating and may supersede LMP-based estimates when the discrepancy is significant (typically > 5–7 days in the first trimester). After the first trimester, ultrasound dating becomes less precise for establishing gestational age. This calculator provides an educational estimate only. The EDD should always be confirmed or refined by early ultrasound when available, and clinical decisions should be based on the most reliable dating method available for the individual patient.
Specialty
ObstetricsCategory
Obstetrics & GynecologyDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-07
Formula
Keywords
Normal Range
Typically around 40 weeks from LMP
Estimates the expected date of delivery (EDD) by adding 280 days to the first day of the last menstrual period (LMP), per the conventional Naegele calculation.
Expected date of delivery = last menstrual period date + 280 days, corresponding to the Naegele rule of a 40-week gestation.
First day of the last menstrual period, the starting date to which 280 days are added.
The calculator returns a single estimated date of delivery, approximately 40 weeks (280 days) after the LMP. The estimate assumes a regular 28-day cycle; longer or shorter cycles shift the true due date. The result is an estimate — first-trimester ultrasound dating is considered more reliable and may supersede LMP dating when the dates differ meaningfully.
Estimating the due date is one of the first steps in prenatal care, guiding the scheduling of screening, monitoring, and delivery planning; LMP-based estimation is the conventional starting point, with ultrasound used to confirm or correct the date.
A patient's first day of last menstrual period was January 1, 2026.
Inputs:
Expected Result: EDD = 2026-01-01 + 280 days = 2026-10-08 — reported as the estimated date of delivery.
First-trimester ultrasound dating by crown-rump length is considered more reliable than LMP dating and may supersede the LMP-based estimate when the discrepancy is typically greater than 5 to 7 days. After the first trimester ultrasound becomes less precise for establishing gestational age, so the earliest reliable dating available for the patient should guide care.
The Naegele rule assumes a regular 28-day cycle with ovulation about 14 days before the next menses. Longer or shorter cycles shift the true due date, and the calculator does not incorporate cycle length or regularity, or assisted-reproduction dating. When the cycle is irregular or the LMP is uncertain, the estimate should be confirmed or corrected by early ultrasound.
No. The EDD is a dating estimate only; it does not assess viability, fetal well-being, or the timing of obstetric interventions, all of which require clinical assessment. The calculator adds 280 days to the LMP date and returns an anticipated date of birth that should be confirmed or refined with early ultrasound when available.
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. The expected date of delivery is an estimate based on the LMP and a 28-day cycle assumption; ultrasound dating should be used to confirm or correct the date when clinically indicated.