Can a dating scan for pregnancy be as much week off when done before 10 weeks?

Dating scans conducted before 10 weeks of gestation are highly accurate in determining the gestational age of a fetus. While there may be some variation in measurements, the margin of error is typically not more than a few days. However, it's important to keep in mind that these scans are not meant to provide an exact due date, but rather to estimate it within a reasonable range.

Ultrasound scans are typically able to accurately determine gestational age by measuring the size of the fetus, particularly the crown-rump length (CRL). This measurement is taken from the top of the head to the bottom of the buttocks of the fetus. The CRL is then compared to standard growth charts to estimate the gestational age.

It's worth noting that dating scans done later in pregnancy, such as the anomaly scan performed at around 18-20 weeks, are considered more precise in predicting the due date. These scans also provide additional information about fetal development and can detect any abnormalities or concerns.