Assistant Professor of Radiology Department of Radiology Mashhad Medical Sciences Islamic Azad University, Mashhad, Iran
Gestational age (GA) assignment has a crucial role in proper timing of most obstetric cares, management of many obstetric problems especially SGA, estimation of the due date and even in some legal issues. Conceptional age is popular in embryology and other medical basic sciences, but from a clinical point of view menstrual age is more acceptable. Although the first day of the last menstrual period (LMP) is used as clinical establishment of GA, it could be misleading up to 20% of cases. Measurement of Crown Rump Length (CRL) in 7 to 11 weeks is considered the most accurate index to establish GA. Assignment of GA in the 2nd and 3rd trimester is done by other ultrasonographic parameters such as Head Circumference (HC), Biparietal Diameter (BPD), Femur length (FL) and Abdominal Circumference (AC), but they are less accurate in comparison with CRL in 1st trimester. In the 2nd and 3rd trimester combination of multiple parameters is more accurate than a single parameter.