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

OBJECTIVE: To determine the risk of placental dysfunction associated with different ultrasound-based phenotypes of impaired fetal growth in dichorionic twin pregnancies. DESIGN: Retrospective cohort study. SETTING: Single tertiary centre. POPULATION: (2011-2023) (N = 753). METHODS: 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. MAIN OUTCOME MEASURES: Abnormal placental histopathology known to be associated with FGR. RESULTS: 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). CONCLUSION: 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.