Composite Biometry versus Head Circumference Alone in Second Trimester Dating of Twin Gestations: A Retrospective Cohort Study

Objective To establish the due date in a singleton gestation in the second trimester, U.S. societal guidelines recommend using a composite of the fetal biparietal diameter, head circumference, abdominal circumference, and femur length. In twin pregnancies, evidence supports using either the head circumference or composite biometry of the larger twin. Our objective was to compare the accuracy of these two approaches in determining the true gestational age of twin gestations. Study Design We conducted a retrospective cohort study of dichorionic, diamniotic gestations conceived with assisted reproductive technologies between 2006 and 2016, excluding fetuses with major anomalies that could impact accurate dating, pregnancies complicated by loss of one or both twins, or higher-order multiple gestations reduced to twins. The earliest second-trimester ultrasound study for each subject was reviewed and the gestational age was calculated using head circumference alone and composite biometry based on Hadlock's formulas. Paired sample statistics were used to calculate the mean difference in days from the observed versus true gestational age to assess the accuracy of each strategy and represented graphically using Bland-Altman plots. Results A total of 179 subjects with a second-trimester ultrasound were identified that met criteria over the 10-year period. Ultrasounds were performed at a mean gestational age of 19.9 days (range: 14.43-26.14). Use of head circumference alone resulted in a mean absolute difference of 3.11 days from the true gestational age (standard deviation [SD]: 4.51, standard error mean [SEM]: 0.34). Composite biometry resulted in a mean absolute difference of 1.74 days (SD: 3.60; SEM: 0.27). The difference of approximately 1 to 2 days was significant (p < 0.001). Conclusion In our twin population, the use of composite biometry of the larger twin more accurately approximates gestational age compared with head circumference alone, though the difference between the two approaches is modest. Key Points Head circumference or composite biometry is used to date twins. In our study, composite biometry improved accuracy. The difference between the two was modest (1-2 days).