Correlation of Biparietal Diameter (BPD) with Menstrual Gestational Age in the Third Trimester: A Cross-Sectional Study

Abstract

Background: Gestational age (GA) is a key factor to optimal prenatal care and proper GA assessment is essential. Biparietal diameter (BPD) is one of the commonly used ultrasonographic measurements to estimate GA, but its accuracy decreases with advancing gestation. Objective: To assess the correlation between BPD and menstrual GA, based on last menstrual period (LMP) in the 3rd trimester and to investigate the effect of fetal lie, placental position and maternal age on the discrepancy between the two. This was a cross-sectional study performed in a private ultrasound clinic in Al-Kut, Iraq from April to July 2024. 75 singleton pregnant women were enrolled at 21-40 weeks of gestation. BPD was measured using a 3.5 MHz curvilinear probe (Philips HD 11XE) in the transthalamic plane. The differences between USG and LMP were classified into six types (A–F). Results: Most of the cases (39%) had a difference of between 1 and 7 days (Category B) and 29% had a difference of between 8 and 14 days (Category C). In general, age of the mother was correlated with the extent of the discrepancy between the two measurements, with the best correlation found in women aged 21-30 years. Anterior position of placenta produced the least variability while posterior position produced more measurement error. The most accurate BPD estimates were obtained from the case of longitudinal cephalic presentation. Conclusion: BPD is a good GA estimation tool during the third trimester, but its performance reduces as gestation increases. It is strongly recommended that BPD be used in conjunction with other complementary biometric indices as well as maternal/fetal factors.