Estimate due date, gestational age, and trimester from your last menstrual period (Naegele's rule).
A free, in-browser pregnancy due date calculator. Enter the first day of your last menstrual period (LMP) and your average cycle length, and get your estimated due date, current gestational age, and trimester — calculated with Naegele's rule and adjusted for cycles that are longer or shorter than 28 days. Everything runs locally in your browser; no dates or personal information are uploaded.
Enter your LMP
Pick the first day of your last menstrual period. This is the starting point for Naegele's rule — not the day your period ended.
Set your cycle length
Default is 28 days. If your typical cycle is longer or shorter, adjust it between 20 and 40 days. The tool shifts your due date by the difference from 28.
Read your result
The estimated due date, current gestational age in weeks + days, and the trimester update immediately.
LMP-based estimation works well when you remember the date precisely and your cycles are regular. ACOG considers first-trimester ultrasound more accurate, especially before 14 weeks, and recommends revising the due date if ultrasound and LMP differ by more than a week in the first trimester.
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is 32 days you likely ovulate around day 18, so conception happens about 4 days later than the textbook assumes — pushing your due date 4 days later too.
Rarely — only about 5% of babies are born on their estimated due date. About 80% arrive within two weeks of it. Birth between 37 and 42 weeks is considered full term.
No. Your LMP and cycle length stay in your browser. Nothing is sent to a server or stored in a database.
The estimated due date (EDD) is calculated using Naegele's rule, first formalised by German obstetrician Franz Karl Naegele in 1830: EDD ≈ LMP + 280 days (40 weeks). The rule assumes a 28-day menstrual cycle with ovulation on day 14; for other cycle lengths, the due date is typically shifted by the difference from 28 days, because conception — not menstruation — is the true anchor of gestation. The American College of Obstetricians and Gynecologists (Committee Opinion 700, 2017) endorses LMP for initial dating but recommends first-trimester ultrasound measurement of crown-rump length as the most accurate method, and advises re-dating when LMP and ultrasound disagree by more than 5–7 days before 14 weeks. Gestational age is reported in completed weeks + days from the LMP; trimesters follow the conventional cut-offs of < 13 weeks, 13 to < 27 weeks, and ≥ 27 weeks.