A due date is a calendar convention: the first day of the last menstrual period plus 280 days, or 40 weeks. It marks the center of the period in which birth is expected, not the expected day of birth. Pregnancies ending between three weeks before the date and two weeks after it are classified as term.

The due date is the reference point for every gestational-age calculation during the pregnancy. Professional guidance recommends establishing it as early and as accurately as possible and not revising it afterward.

The term range

The American College of Obstetricians and Gynecologists divides term into four bands. Early term runs from 37 weeks 0 days to 38 weeks 6 days. Full term runs from 39 weeks 0 days to 40 weeks 6 days. Late term runs from 41 weeks 0 days to 41 weeks 6 days, and post-term begins at 42 weeks 0 days. The due date falls in the middle of the full-term band, which is itself a two-week interval within a five-week range.

The distribution of births within that range is a separate question. Smith (2001) analyzed pregnancies that ended without a scheduled delivery and found a median of 284 days for first births and 282 days for later births, both slightly later than the 280-day convention. Half of the first pregnancies in that group continued past 40 weeks 4 days.

Sources of variation

Part of the variation is measurement. The 280-day rule assumes ovulation on day 14 of a 28-day cycle, and cycle length varies. A period date may be recalled incorrectly. Ultrasound dating is more precise early in pregnancy, but a first-trimester scan still carries an error of several days. Napolitano and colleagues (2014) found that the published crown-rump length charts used for that dating differ from one another.

The remainder is biological. Jukic and colleagues (2013) followed 125 naturally conceived pregnancies in which ovulation had been identified from hormone measurements, which removes dating error. After excluding preterm births and pregnancies with related medical conditions, gestational length still ranged from 247 to 284 days after ovulation, a span of 37 days. That variation cannot be reduced by a calculator.

How the due date is established

Committee Opinion 700 sets a priority order. A first-trimester ultrasound is the most accurate method, and when it differs from the period-based date by more than a specified margin, the ultrasound date is used. The margin increases with gestational age: 5 days before 9 weeks, 7 days from 9 weeks to under 14 weeks, and larger margins later. Once a due date is established by these rules, later scans do not revise it, because later measurements reflect fetal growth as well as fetal age.

Known conception dates take priority over the period date. For IVF, the transfer date and the embryo's age at transfer determine the due date directly. For tracked ovulation, the ovulation date plus 266 days is used.

The due date calculator applies this arithmetic to each date entered and shows every candidate date with its difference from the selected one. It does not average dates. When candidates differ by more than seven days, the calculator reports the difference and recommends discussing it with the care team. If a clinician has already established a due date, entering it takes priority over the other sources, because that date is the basis for care decisions.

Using the date

The due date is the reference for gestational age, which determines the timing of appointments, screening windows, and the term bands above. For the question of when labor is likely to begin, use the labor calculator. It starts from the same date and returns a range and weekly probabilities derived from the variation described here.

In Smith's cohort, continuing past the due date was the more common outcome for first pregnancies.