Association between the ratio of placental weight to birthweight and the risk of neurodevelopmental delay in 3-year-Olds: The Japan environment and Children's study

Introduction: The ratio of placental weight to birthweight (PW/BW ratio) is well known as a simple indicator of the prenatal intrauterine environment and placental functioning. We assessed the impact of PW/BW ratio on the risk of neurodevelopmental delay in 3-year-olds. Methods: We used data of 71 205 mother-child pairs enrolled in the Japan Environment and Children's Study. Low PW/BW ratio was defined as a PW/BW ratio below the 10th percentile of the study population, high PW/BW ratio was defined as above the 90th percentile, and normal PW/BW ratio was defined as between the low and high PW/BW ratio. Neurodevelopment was assessed using the Japanese translation of the Ages and Stages Questionnaires, third edition. Associations between PW/BW ratio and risk of developmental delay were examined using multivariable models. Results: Compared with boys in the normal PW/BW group, boys in the high PW/BW group had higher risk for developmental delays in all domains except fine motor skills (communication: adjusted risk ratio [aRR], 1.17; 95% confidence interval [CI], 1.04-1.33; gross motor skills: aRR, 1.28; 95% CI, 1.13-1.46; problem solving: aRR, 1.20; 95% CI, 1.09-1.31; personal-social: aRR, 1.26; 95% CI, 1.10-1.43), and boys in the low PW/BW group also had higher risk for developmental delays in some domains. For girls, there was almost no association between PW/BW ratio and developmental delay. Discussion: An unbalanced PW/BW ratio, especially high PW/BW ratio, might indicate intrauterine suboptimality, which affects child neurodevelopment in a sex-specific manner.