Risk of Placental Pathology Across Ultrasound-Defined Phenotypes of Impaired Fetal Growth in Dichorionic Twins: A Retrospective Cohort Study.

To determine the risk of placental dysfunction associated with different ultrasound-based phenotypes of impaired fetal growth in dichorionic twin pregnancies. Retrospective cohort study. Single tertiary centre. Patients with dichorionic twin pregnancies delivered at ≥ 320/7 (2011-2023) (N = 753). Each twin fetus was classified into a distinct exposure group based on combinations of fetal weight percentile (categorised as AGA, mild SGA, or severe SGA) based on singleton- or twin-specific growth chart, weight discordance and Doppler. Abnormal placental histopathology known to be associated with FGR. The rate of the primary outcome in the reference group (AGA, concordant growth, normal Doppler) was 12.8%. When using a singleton chart, isolated discordance and mild SGA were not associated with the primary outcome. Only isolated severe SGA (28.8%, aRR 2.47[1.52-4.03]), followed by severe SGA with discordant growth (41.9%, aRR 3.53[2.15-5.79]) and abnormal Doppler (54.7%, aRR 4.33[2.90-6.48]) showed significant associations with the primary outcome. In contrast, when using a twin-specific chart, both isolated discordance (32.4%, aRR 2.63[1.46-4.73]) and mild SGA (28.2%, aRR 2.43[1.30-4.51]) were associated with the primary outcome. We propose three risk categories for late-onset placenta-mediated FGR based on the likelihood of the primary outcome: (1) possible small increase (< 2-fold); (2) moderate increase (2-3 fold); and (3) highest risk (> 4-fold). The risk categories proposed in the current study can serve as a framework to inform the design of future trials and contribute to the development of guidelines for managing twin pregnancies affected by late-onset FGR.