Adherence to the Planetary Health Diet Index and Fetal Body Composition.

Importance: The Planetary Health Diet (PHD), introduced by the EAT-Lancet Commission in 2019, emphasizes a plant-based diet. Several cohorts have assessed adherence using the PHD Index (PHDI), but evidence is limited on whether maternal periconceptional and early pregnancy adherence is associated with fetal growth. Objective: To examine the association of maternal PHDI adherence in early pregnancy with longitudinal 2-dimensional and 3-dimensional fetal biometric measures. Design, Setting, and Participants: Secondary analysis of a prospective cohort of pregnant women with singletons (July 2009 to January 2013) at 12 US clinical sites. Women with preexisting health conditions were excluded. Participants had data on fetal body composition and organ volumes (April 2016 to September 2019). Analyses were conducted June 2024 to August 2025. Exposure: PHDI adherence categorized as low, moderate, or high, assessed by the Food Frequency Questionnaire between 8 to 13 weeks' gestation. Main Outcomes and Measures: Estimated fetal weight (EFW), head circumference (HC), humerus and femur lengths, abdominal circumference and area, abdominal subcutaneous tissue thickness, fractional thigh and arm volumes (total, lean, and fat), midthigh areas, ratio of fractional fat thigh volume to fractional thigh volume, and ratio of midthigh fat area to midthigh area, measured up to 5 times between 15 to 42 weeks' gestation. Trajectories were modeled using linear mixed-effects models. Results: Analyses included 1464 women. Mean (SD) maternal age was 28.1 (5.6) years, and mean (SD) gestational age at delivery was 39.2 (1.7) weeks. High vs low PHDI adherence was associated with larger EFW at 32 to 40 weeks (32 weeks: difference, 35 g; 95% CI, 22 to 49 g; 40 weeks: difference, 165 g; 95% CI, 108 to 223 g), larger HC at 37 to 39 weeks (37 weeks: difference, 1.89 mm; 95% CI, 1.10 to 2.69 mm; 39 weeks: difference, 2.44 mm; 95% CI, 1.47 to 3.42 mm), smaller fractional lean arm volume at 34 to 37 weeks; (34 weeks: difference, -0.40 cm3; 95% CI, -0.56 to -0.25 cm3), larger fractional fat arm volume at 28 to 29 weeks; (28 weeks: difference, 0.23 cm3; 95% CI, 0.13 to 0.34 cm3), and larger abdominal area at 25 to 26 weeks (25 weeks: difference, 62.2 mm2; 95% CI, 34.4 to 90.0 cm3). Moderate adherence showed similar patterns. Conclusions and Relevance: In this cohort study of pregnant women, higher maternal PHDI adherence in early pregnancy was associated with greater fetal adiposity and reduced lean tissue, suggesting potential implications for offspring metabolic health.