Placental abnormalities differ between small for gestational age fetuses in dichorionic twin and singleton...

Twin fetuses grow slower during the third trimester compared with singletons. However, the extent to which the relative smallness of twins is the result of placenta-mediated factors similar to those associated with fetal growth restriction in singletons remains unclear. Our aim was to address this question by comparing placental findings between small for gestational age (SGA) twins and SGA singletons.

Retrospective cohort study of all SGA non-anomalous newborns from singleton and dichorionic twin pregnancies in a single tertiary referral center between 2002 and 2015. SGA was defined as birth weight <10th percentile for gestational age according to sex-specific national reference charts. Placental findings were compared between SGA twins and SGA singletons and were classified into lesions associated with maternal vascular malperfusion, fetal vascular malperfusion, placental hemorrhage and chronic villitis.

A total of 532 SGA twins and 954 SGA singletons met the inclusion criteria. SGA twins had a higher mean placental weight (371 ± 103 g vs. 319 ± 107, p < 0.001) and a lower fetal-placental ratio (6.0 ± 2.5 vs. 6.7 ± 3.2, p < 0.001) compared with SGA singletons. Compared with SGA singletons, SGA twins were less likely to have any placental pathology (aOR 0.37, 95%-CI 0.29-0.46), hypercoiled cord (aOR 0.45, 95%-CI 0.33-0.61), placental weight<10th% (aOR 0.13, 95%-CI 0.08-0.20), maternal vascular malperfusion pathology (aOR 0.24, 95%-CI 0.18-0.30) and fetal vascular malperfusion pathology (aOR 0.62, 95%-CI 0.48-0.82). By contrast, SGA twins had higher odds of a marginal or velamentous cord insertion compared with SGA singletons (aOR 13.82, 95%-CI 10.44-18.30). Similar significant associations were observed in subgroups of SGA fetuses with a birth weight below the 5th and 3rd percentile for gestational age.

Our findings illustrate that the mechanisms underlying reduced fetal growth in dichorionic twins differ from those involved in singletons, and may provide support to the hypothesis that smallness in dichorionic twins may be more benign than in singletons.