Crown-rump length (CRL) is the most accurate sonographic measurement for determining gestational age, typically used between 6 and 13 weeks. It measures the longest straight-line distance from the top of the fetal head (crown) to the bottom of the torso (rump), excluding the limbs. A CRL measurement in the first trimester carries an accuracy of approximately ±5–7 days. After 13–14 weeks, head and abdominal biometry provide more reliable gestational age estimates.