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Twin fetuses grow slower during the 3rd trimester compared with singletons. Whether the relative smallness of twins is the result of placental dysfunction (as in singletons) or whether it reflects a benign adaptive response to the competitive uterine environment is yet unclear. This question is important as it determines whether small for gestational age (SGA) twins need to be managed similar to SGA singletons. We previously reported that SGA twins have a lower rate of abnormal placental pathology compared with SGA singletons, supporting the hypothesis that the smallness of twins may be more benign than in singletons. In the current study we aimed to further test this hypothesis by comparing the rate of Doppler abnormalities, as a specific antenatal measure for placental dysfunction, between SGA twins and SGA singleton fetuses.
We conducted a retrospective cohort study of SGA dichorionic twin and singleton fetuses (birth weight <10th percentile for gestational age) born in a single tertiary center in Toronto, Ontario between 2010-2017 and for which Doppler data was available. Doppler findings in the most recent ultrasound report prior to birth were compared between SGA twins and singletons. Doppler abnormalities included elevated umbilical artery pulsatility index (UA-PI) >95th%, absent/reverse diastolic flow (AREDF) in the umbilical artery, low middle cerebral artery pulsatility index (MCA-PI)<5th% and low cerebroplacental ratio (CPR)<5th%.
Overall 1,121 SGA singleton and 375 SGA twin fetuses were included. Twins were born earlier than singletons (35.8±2.3 weeks vs. 37.5±3.4 weeks, p<0.001). In the overall cohort, the rate of Doppler abnormalities was similar for SGA twins and SGA singletons, except for a higher rate of AREDF in the singletons group (4.3% vs. 1.1%, p=0.003) (Table 1). These findings persisted in the subgroup of SGA fetuses born at <34 weeks (Table 2). However, for SGA fetuses born at 34-37 weeks, twins had a significantly lower rate of any Doppler abnormality (22.3% vs. 33.1%, p=0.007), MCA-PI<5th percentile (14.8% vs. 23.6%, p=0.012) and CPR<5th percentile (17.0% vs. 26.0%, p=0.014) (Table 2). The number of twins born after 37 weeks was too small to allow for proper interpretation.