Which measurement is commonly used in obstetric ultrasound to estimate fetal age?

Crown-rump length (CRL) is the most commonly used measurement in obstetric ultrasound to estimate fetal age, particularly during the first trimester. This measurement represents the distance from the top of the fetus's head (the crown) to the bottom of its buttocks (the rump). CRL is highly reliable for determining gestational age because the growth rate during early pregnancy is relatively uniform among fetuses.

This uniform growth allows healthcare professionals to accurately assess how far along the pregnancy is based on the CRL measurements taken during the ultrasound. The accuracy of CRL in estimating gestational age is particularly pronounced in the first trimester when fetal growth is rapid and consistent.

In contrast, while other measurements such as head circumference, abdominal circumference, and femur length may provide additional information about fetal growth and development, they become more relevant later in the pregnancy. These measurements can be influenced by various factors, including genetics, maternal health, and dietary factors, which can affect the estimation of fetal age. Thus, CRL remains the preferred choice for early gestational age estimation.