The accuracy of sonographic fetal head circumference in twin pregnancies.

OBJECTIVE: To assess the accuracy of sonographic estimation of fetal head circumference in twin gestations.

METHODS: A retrospective analysis of sonographic evaluations of twin gestations >34 weeks, performed within 7 days of delivery, in a single university-affiliated medical centre. Sonographic head circumference was compared with neonatal head circumference. Measures of accuracy included systematic error, random error, proportion of estimates within 5% of neonatal head circumference, and reliability analysis. Accuracy of sonographic head circumference was compared between the first and second twin.

RESULTS: Overall, 103 twin gestations were evaluated at a median of 4 days before delivery. The majority of twins were dichorionic-diamniotic (83%). Median gestational age at delivery was 37 weeks, with a median birthweight of 2645 grams for the first twin and 2625 grams for the second twin. For all fetuses, median sonographic head circumference was lower than the neonatal head circumference (first twin: 317.5 vs. 330 mm; second twin: 318.4 vs. 330 mm, P > 0.05 for both). Measures of accuracy showed no significant difference between first and second twin. There was no difference in the number of sonographic head circumference evaluations that were within 5% of the neonatal head circumference between fetuses (64% for both twins). Cronbach α value was higher for the second twin (0.746 vs. 0.613), suggesting higher accuracy for head circumference measurement for the second twin.

CONCLUSION: In our cohort, sonographic head circumference underestimated postnatal head circumference. Accuracy measurements were not significantly different between the first and second twin.