EDD Calculator — Estimated Date of Delivery & Gestational Age

About

The Estimated Date of Delivery (EDD), also known as the estimated due date, is calculated using Naegele's rule, which adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). This rule was developed by the German obstetrician Franz Karl Naegele (1778–1851) and remains the most widely used method for calculating due dates. The 280-day duration is based on the average length of human gestation from the first day of the LMP, assuming a regular 28-day menstrual cycle. While Naegele's rule provides a convenient and standardized estimate, it assumes ovulation on day 14 of the cycle and does not account for cycle length variations. First-trimester ultrasound (crown-rump length) is considered the most accurate method for pregnancy dating and may adjust the EDD by up to ±7 days.

Formula

EDD = LMP + 280 days = LMP + 40 weeks

Naegele's rule calculates the estimated date of delivery by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). The calculation assumes a regular 28-day menstrual cycle with ovulation occurring on day 14. For women with longer cycles (>28 days), the EDD may be adjusted by adding the difference in cycle length. For example, for a 35-day cycle, add 7 additional days (35 - 28 = 7) to the calculated EDD. The gestational age at any point in pregnancy is calculated as the number of weeks and days elapsed since the LMP. The pregnancy is divided into three trimesters: First Trimester (1-13 weeks), Second Trimester (14-27 weeks), and Third Trimester (28-42 weeks). Term pregnancy is defined as 37-42 weeks, with early term (37-38 weeks), full term (39-40 weeks), late term (41 weeks), and post-term (≥42 weeks).

Score Interpretation

Accurate determination of gestational age and estimated date of delivery is fundamental to modern obstetric care. The EDD guides the timing of antenatal screening tests (first-trimester screening at 11-14 weeks, anatomy ultrasound at 18-22 weeks, glucose tolerance test at 24-28 weeks), determines when a pregnancy is considered term (37-42 weeks), and informs decisions regarding labor induction for post-term pregnancies (≥42 weeks). Naegele's rule, first described in the early 19th century, remains the standard for EDD calculation despite its known limitations. The WHO, ACOG, and NICE all recommend first-trimester ultrasound for the most accurate pregnancy dating. Gestational age assessment is critical for managing preterm labor, determining fetal viability thresholds, and scheduling interventions. Incorrect dating can lead to unnecessary inductions, inappropriate management of post-term pregnancy, and neonatal complications.

Reference Ranges