Comparison between Femur Length Measurements by ultrasound and Last Menstrual Period for Gestational Age Assessment: Influencing Factors in the Third Trimester

Abstract

Background: Measurement of the femur length (FL) is one of the basic measurements used in obstetrics to determine gestational age (GA), especially during the second and third trimesters of pregnancy, by ultrasonography. However, its accuracy is influenced by a number of clinical and technical factors such as gestational age, fetal lie, position of the placenta and maternal age. These modulating factors should be well understood to enhance the reliability of fetal biometry and to help detect fetal growth abnormalities like intrauterine growth restriction (IUGR) early. Methods The study was conducted in third trimester pregnant women who attended private obstetric clinics, in Wasit and Baghdad governorates in Iraq, during a period of September 2023 – September 2024. The length of the femur was measured by ultrasound machine Philips HD11 (3.5 MHz convex probe) in accordance with the standard obstetric biometry protocol. LMP derived gestational age discrepancies were divided into 6 categories: A = no discrepancy; B = 1-7 days discrepancy; C = 8-14 days discrepancy; D = 15-21 days discrepancy; E = 22-28 days discrepancy; F = 29-35 days discrepancy. Results: Most of the cases (46.67%) belonged to Group B (1-7 days). The most common fetal lie was longitudinal cephalic (84%), which correlated with the most consistent FL measurements. Anterior placental position was the most common (50.67%) and showed the widest distribution of discrepancy categories. The highest consistency in measurement was found in women aged 21-30 years. Conclusions: The fetal lie, placental position, gestational age and maternal age are significant factors affecting the accuracy of FL measurement in the third trimester. These variables must be documented systematically during obstetric ultrasound and other biometric variables are recommended for use in improving accuracy in the estimation of GA.