A first-trimester ultrasound dates a pregnancy to within about five to seven days. Committee Opinion 700 from the American College of Obstetricians and Gynecologists describes crown-rump length measurement before 14 weeks 0 days as the most accurate dating method. It recommends that such a scan replace a period-based date when the two differ by more than a set margin.
The due date calculator accepts the gestational age from a scan report and converts it to a due date. It labels a first-trimester scan as a high-certainty source and ranks it above the period date and tracked ovulation.
How the measurement works
Crown-rump length is the length of the embryo or fetus from the top of the head to the bottom of the torso, measured in millimeters. In the first trimester, size is closely related to age because individual variation in growth is small at that stage. A chart converts the measurement to a gestational age, and the due date follows by adding the remaining days to 280.
The relationship weakens as pregnancy progresses. By the second trimester, differences in growth between individual fetuses are large enough that size is a poorer guide to age, and the dating error rises to about 10 to 14 days. In the third trimester it is about three weeks.
Sources of error
Measurement error is one source. Crown-rump length depends on the fetus being in a neutral position and on the plane of the image, and small differences in technique produce differences of a few days.
Chart differences are another. Napolitano and colleagues (2014) reviewed the published crown-rump length charts. The charts were built from different populations with different methods. The same measurement can give gestational ages that differ by a few days depending on which chart is used. The calculator asks for the report's stated gestational age rather than the raw measurement so that the chart the clinic used is the one applied.
Biological variation in the interval between ovulation and implantation contributes as well. Mahendru and colleagues (2016) compared dating by period date, crown-rump length, ovulation date and implantation date, and found that even ovulation-based dating leaves a residual spread in gestational length that no measurement can remove.
When the scan overrides the period date
Committee Opinion 700 sets the margins. Before 9 weeks 0 days, the ultrasound date is used when it differs from the period date by more than 5 days. From 9 weeks 0 days to 13 weeks 6 days, the margin is 7 days. From 14 to 15 weeks 6 days it is 7 days. From 16 to 21 weeks 6 days it is 10 days. From 22 to 27 weeks 6 days it is 14 days, and from 28 weeks it is 21 days. A later scan is used to change the date only when no earlier scan exists.
Once a due date has been established by these rules, it is not revised by later scans. Later measurements reflect how the fetus is growing, and a change in the due date would obscure that information.
What the calculator does with a scan
The calculator converts the report's gestational age to a due date, compares it with any other dates entered, and shows the difference. When the scan and period dates differ by more than seven days, it recommends discussing the difference with the care team. It does not change the established date on its own. That decision belongs to the clinician who has the full record.
A first-trimester scan is the best single source for dating, and its five-to-seven-day error is small relative to the five-week range within which term births occur. It does not make the due date a prediction of the day of birth. The labor calculator addresses that question separately, starting from whichever due date the care team has established.