Due date based on ultrasound: when a scan changes the date

A first-trimester scan measures the embryo from head to rump

ACOG Committee Opinion 700, "Methods for Estimating the Due Date" (2017), states that ultrasound measurement of the embryo or fetus in the first trimester, up to and including 13 weeks and 6 days, is the most accurate method to establish or confirm gestational age. Dating uses the crown-rump length (CRL), which has an accuracy of 5 to 7 days up to 13 weeks and 6 days, and it is more accurate the earlier in the first trimester the scan is done. The opinion recommends the mean of three CRL measurements where possible and does not recommend mean sac diameter for estimating the due date. Beyond a CRL of 84 mm, about 14 weeks and 0 days, other measurements are used because CRL accuracy falls (ACOG, 2017). ACOG's patient guidance adds that before about 7 weeks the crown and rump cannot be seen, so the greatest length of the embryo is measured instead (ACOG, "How Your Fetus Grows During Pregnancy", 2024).

The scan date replaces the last-period date only past a threshold

Where the last-period date is known, ACOG's opinion sets the size of the gap between the scan date and the last-period date that should change the due date. The gap depends on how far along the pregnancy is when the scan is done (ACOG, 2017, Table 1). The scan replaces the last-period date when the difference is more than:

The table is the rule the guideline gives clinicians, and it is not a way to check a scan of your own: ACOG says smaller differences can also justify a change, depending on how early the scan was done and on the clinician's view of how reliable the last-period date is (ACOG, 2017). The due date calculator on this site applies these thresholds when you enter both a last-period date and a scan, and shows when it has changed the date.

Later scans rarely move a date set in the first trimester

ACOG says that, with rare exceptions, a due date that a first-trimester scan has confirmed should not be adjusted by a second-trimester scan. Dating in the second and third trimesters is less exact: accuracy is 7 to 10 days from 14 weeks 0 days to 21 weeks 6 days, 10 to 14 days from 22 weeks 0 days to 27 weeks 6 days, and 21 to 30 days from 28 weeks 0 days. Because a small fetus in the third trimester may be growth restricted rather than younger than expected, ACOG says management decisions based on third-trimester ultrasound alone are especially problematic (ACOG, 2017).

If a person is unsure of the last period, ACOG says the date should rest on ultrasound, ideally at or before 13 weeks and 6 days, and that the earliest CRL measurement is treated as the most reliable. If the pregnancy followed IVF, the date set from the embryo's age and the transfer date is used and a scan does not override it (ACOG, 2017).

A scan date is still an estimate

A retrospective study of 18,708 women in Australia found that only 5 in 100 births occurred on the estimated date, and about 66 in 100 within 7 days, with little difference between dating scans done at different gestations. The scan at 11 to 14 weeks performed as well as scans at other times (Khambalia et al., International Journal of Gynecology & Obstetrics, 2013). See estimated due date for what that means for planning.

Dating scans in the UK and US

In the UK, NICE guideline NG201 advises offering a scan between 11 weeks and 2 days and 14 weeks and 1 day to determine gestational age, to detect a multiple pregnancy and, if the person chooses, to screen for Down's, Edwards' and Patau's syndromes (NICE, 2021). The NHS calls it the 12-week or dating scan, and says it checks how many weeks pregnant you are and works out the due date, whether you are expecting more than one baby, that the baby is growing in the right place, and the baby's development. It usually takes about 20 minutes, and you may need a full bladder (NHS, "12-week scan", 2023). The NHS states that scans are offered and that you do not have to have them, and that antenatal care continues as normal if you decline (NHS, "Ultrasound scans in pregnancy", 2023).

In the US, ACOG's opinion describes how to use an ultrasound examination to set or confirm the date and calls a pregnancy without one before 22 weeks and 0 days suboptimally dated (ACOG, 2017). Ask your own obstetric provider when a scan will be done. The pregnancy timeline places the scan among the other first-trimester appointments.

This guide is information, not a diagnosis or a recommendation of treatment, and it is no substitute for your own clinician. See the medical disclaimer.