Four kinds of information can establish a due date. They are the first day of the last menstrual period, an ultrasound measurement, a known ovulation or fertilization date, and an IVF transfer date. Each is converted to a due date by a fixed rule.
When more than one is available, professional guidance selects one by a priority order rather than combining them.
The due date calculator applies each rule to the dates entered and reports every candidate.
Last menstrual period
The due date is the first day of the last period plus 280 days. The rule assumes ovulation on day 14 of a 28-day cycle, so the calculator adds the difference between the user's usual cycle length and 28 days. A 32-day cycle adds four days.
The method depends on accurate recall of the period date and on regular cycles. Ovulation timing varies within and between people even in regular cycles. Wilcox and colleagues (1995) documented that the fertile window spans about six days ending on the day of ovulation. A period-based date therefore carries an uncertainty of about a week in ordinary circumstances and more with irregular cycles.
Ultrasound
An ultrasound report states a gestational age on the scan date, derived from fetal measurements. The calculator converts it by subtracting the reported gestational age from the scan date and adding 280 days. The user enters the report's figure. The calculator does not apply crown-rump length formulas itself, because those depend on the chart and technique used.
First-trimester measurement of crown-rump length is the most accurate dating method available. Committee Opinion 700 gives an error of about 5 to 7 days for scans before 14 weeks. Napolitano and colleagues (2014) reviewed the published crown-rump length charts and found that they differ from one another. The same measurement can yield gestational ages a few days apart depending on the chart. Accuracy declines with gestational age, because fetal size varies more between individuals as pregnancy progresses.
Ovulation or fertilization
When the ovulation date is known from hormone testing or another reliable method, the due date is that date plus 266 days. Mahendru and colleagues (2016) compared dating based on period date, crown-rump length, ovulation date and implantation date. Ovulation-based dating removes the cycle-length part of the error. What remains is the biological variation in gestational length itself.
Tracked ovulation from an app or calendar is less reliable than a hormone-confirmed date, and the calculator ranks it below a first-trimester ultrasound for that reason.
IVF and other assisted conception
For embryo transfer, the due date is the transfer date minus the embryo's age at transfer plus 266 days. A day-5 transfer on March 1 gives a due date of November 17. This is the most precise method, because fertilization occurred on a known date.
For egg retrieval or intrauterine insemination, the calculator uses the procedure date as a conventional ovulation anchor and adds 266 days. It labels the result as an anchor rather than a known fertilization date. A clinic-assigned due date, when one exists, takes priority.
Priority when methods disagree
Committee Opinion 700 establishes the order. An IVF transfer date or known fertilization date ranks first. A first-trimester ultrasound ranks next, and it replaces a period-based date when the two differ by more than 5 days before 9 weeks or 7 days from 9 to under 14 weeks. A reliable period date ranks above tracked ovulation and above second-trimester ultrasound. Once a due date is established, later scans do not revise it.
A due date already established by a clinician outranks every other entry, because it is the date on which care decisions are based.
The calculator shows each candidate date with its difference from the selected one. When candidates differ by more than seven days, it recommends raising the difference with the care team. It does not average dates, because an average of two methods with different error characteristics is not a supported estimate.
Conception timing
From any selected due date, the calculator also reports a conventional conception estimate 266 days before it. The estimate is precise only when fertilization is known. For a period-based or ultrasound-based date, actual conception could have occurred several days earlier or later, and the calculator cannot attribute a pregnancy to a particular day or event.