How Pregnancy Due Dates Are Estimated (and Why the Clinical Date May Differ)
July 24, 2026
A pregnancy due date is an estimated due date (EDD), not a prediction of the exact day labor will begin. A browser calculator can reproduce common date arithmetic, but the date used for prenatal care should come from the clinical record because dating depends on which information is available and how reliable it is.
The common LMP starting estimate
A traditional starting calculation counts 280 days — 40 weeks — from the first day of the last menstrual period (LMP). This is often called Naegele’s rule. It is useful because an LMP date may be available before an ultrasound, but it relies on assumptions about cycle timing that do not fit every pregnancy.
If the menstrual history is uncertain, cycles are irregular, or assisted reproduction was used, the clinical dating process may rely more heavily on other information. The important distinction is simple: the calculator performs calendar arithmetic; the prenatal team establishes the EDD used for care.
Why ultrasound can change the picture
Ultrasound estimates gestational age from fetal measurements rather than simply counting from a menstrual date. First-trimester ultrasound is the most accurate ultrasound method for establishing or confirming gestational age. Dating uncertainty becomes wider later in pregnancy, which is why a later scan should not be used to re-date a pregnancy casually or by comparing it with an online calculator.
When LMP and ultrasound estimates differ enough to matter, clinicians use established dating criteria rather than choosing whichever date feels more plausible. Once an EDD has been selected and documented, later changes should be uncommon and clinically justified.
IVF and other assisted-reproduction pregnancies
For pregnancies conceived through assisted reproductive technology, dating uses the embryo age and transfer date. That information is more specific than trying to reconstruct conception from a period date, so the ART-derived EDD documented by the care team is the date to use for clinical decisions.
A calculator can still be handy for planning, but it should reproduce the known transfer-date arithmetic rather than override the date already supplied by the fertility or prenatal team.
“Term” does not mean one five-week full-term window
The older shorthand that treated 37 through 42 weeks as one broad “full-term” period is too imprecise. ACOG and the Society for Maternal-Fetal Medicine divide the end of pregnancy into more specific categories:
- Early term: 37 0/7 through 38 6/7 weeks
- Full term: 39 0/7 through 40 6/7 weeks
- Late term: 41 0/7 through 41 6/7 weeks
- Postterm: 42 0/7 weeks and beyond
Those labels describe gestational-age categories; they are not a prediction of when an individual pregnancy will deliver and they do not tell you when monitoring, induction or delivery is appropriate for a particular pregnancy.
Using the GentleTools calculator safely
The Due Date Calculator can estimate dates from an LMP, a conception or ovulation date, an IVF transfer, or gestational age recorded at an ultrasound. Use it as a planning reference for dates and week numbers. For screening schedules, pregnancy monitoring, delivery decisions or any question about whether the date should be changed, use the EDD documented by your prenatal care team.
Sources and method. This guide follows the dating and term-pregnancy framework published by the American College of Obstetricians and Gynecologists (ACOG), including Methods for Estimating the Due Date and Definition of Term Pregnancy. GentleTools provides planning and recordkeeping tools, not prenatal care or clinical dating decisions.