Cerebro-placental and umbilico-cerebral ratios in uncomplicated monochorionic twins: Longitudinal references and comparison with singletons

Objectives Monochorionic twins (MC) are at high risk of adverse outcomes and Doppler investigation of umbilical and cerebral flows is mandatory for their surveillance. The cerebro-placental (CPR) and umbilico-cerebral (UCR) ratios are considered non-invasive measures of fetal adaptation to hypoxemia. We aimed to provide longitudinal references for CPR and UCR from 16 to 37 weeks of gestation that are specific for MC twins, and compare these with singleton charts. Methods Longitudinal study of a cohort of consecutive uncomplicated MC twin pregnancies monitored at our unit from 2010 to 2018. The estimated centile curves were obtained estimating the median with fractional polynomials by a multilevel model and the external centiles through the residuals. The comparison with singletons references was made through graphic evaluation. Results One-hundred-fifty-two MC pregnancies were included with a median of 10 longitudinal ultrasounds each. References for CPR and UCR in function of gestational age are presented. Compared to singletons, MC twins showed an earlier and greater circulatory redistribution with lower CPR and higher UCR median values. Conclusions MC twin-specific references for CPR and UCR suitable for serial monitoring are presented. The comparison with singleton references demonstrates substantial differences in the hemodynamic balance that must be considered when interpreting findings in MC twins.