An ultrasonographic estimated fetal weight reference for Japanese twin pregnancies.

The purpose was to establish an estimated fetal weight (EFW) reference for twin pregnancies in Japan and to compare the growth of twins with singletons. We retrospectively investigated Japanese women who delivered live-born twins at our center during the period from 2010 to 2016. The main exclusion criteria were monoamniotic twins, fetal reduction, maternal complications, twin-twin transfusion syndrome, fetal congenital anomalies, and patients with their first visit after 16 weeks' gestation. The EFW was measured longitudinally from 16 to 37 weeks' gestation. We calculated the posterior predictive distribution using hierarchical Bayesian models and determined the EFW corresponding to each Z-score. A total of 364 women (190 dichorionic and 174 monochorionic) were included, and the total number of examinations was 3952. The EFWs of a Z-score of 0 for twins at 20, 28, and 36 weeks' gestation were 308, 1070, and 2294 g, respectively. The EFW of a Z-score of 0 for twins was 98-101% that of singletons until 21 weeks, gradually becoming lower than that of singletons and reaching 90-93% that of singletons after 27 weeks. We established an EFW reference for Japanese twin pregnancies. The EFW of twins is similar to that of singletons until the mid-second trimester, gradually becoming lower than that of singletons and reaching about 90% that of singletons in the third trimester.