Pregnancy due date calculator

Methodology

How this is calculated

Each method produces a due date, and everything else is read from it. From the first day of the last period, the due date is 280 days later. From a conception or ovulation date, it is 266 days later, which is the same rule counted from day 14 of the cycle instead of day 1. From an ultrasound report, it is the scan date plus the days the report's gestational age falls short of 40 weeks. Gestational age today is 280 days minus the days left until the due date, and the trimester and the week 37 to 41 dates follow from that count. The cycle length moves the due date by the cycle's difference from 28 days. That adjustment is derived here, not published: the standard rule assumes ovulation on day 14 of a 28-day cycle, so a longer cycle moves ovulation, and the due date, later by the same number of days. It is the conception method counted from the last period. Committee Opinion No. 700 does not include it, and for an irregular cycle its answer is ultrasound dating.

Where the data comes from

The dating rule and the trimester boundaries come from the American College of Obstetricians and Gynecologists, the American Institute of Ultrasound in Medicine, and the Society for Maternal-Fetal Medicine, Committee Opinion No. 700, Methods for Estimating the Due Date (Obstet Gynecol 2017;129(5):e150-e154). It states: “By convention, the EDD is 280 days after the first day of the LMP.” It notes that this “assumes a regular menstrual cycle of 28 days, with ovulation occurring on the 14th day after the beginning of the menstrual cycle.” It places the first trimester “up to and including 13 6/7 weeks of gestation” and the third at “28 0/7 weeks of gestation and beyond.” The week 37 to 41 dates come from a separate document, cited under their own heading below.

Last updated

September 23, 2026

What this does not account for

Irregular cycles, which a calendar rule handles poorly and ultrasound handles better. Pregnancies after IVF or other assisted reproduction, which are dated from the transfer date and the embryo's age. Twins and other multiples. Whether an ultrasound should replace a date from the last period, which Committee Opinion No. 700 leaves to the clinician with its own thresholds that this tool does not apply. Anything about how a pregnancy is going.