Birth Weight Percentiles and Infant and Child Growth Dynamics.

Importance: Infants classified as small or large for gestational age can have different growth patterns compared with appropriate-for-gestational age counterparts. The association of birth weight percentiles beyond conventional thresholds with early-life growth remains unknown. Objective: To quantify the association of birth weight percentile range with infant and child growth. Design, Setting, and Participants: This is a prospective cohort study of singletons born between 1991 and 2011 in 7 birth cohort studies in Europe, Singapore, and the US and followed up with repeated growth measurements for 10 years. Five European cohorts were used for discovery analysis, and the Singapore and US cohorts were used for replication analyses. Exposures: Birth weight percentiles standardized for sex and gestational age using the INTERGROWTH-21st standards and classified into 10 decile groups, with the middle (fifth and sixth decile groups) as the reference group. Main Outcomes and Measures: The primary outcomes were infant height (centimeters per month) and weight (grams per month) growth velocity at 1, 6, 12, 24 months; body mass index (BMI; calculated as weight in kilograms divided by height in meters squared); age (months or years) at infant BMI peak and childhood BMI rebound; and overweight or obesity at 10 years. Associations were examined using regression models adjusted for sex and birth cohort. Results: The discovery cohort included 36 018 children (mean [SD] gestational age at birth, 39.7 [1.8] weeks; 17 238 girls [48%]). Compared with the reference group, higher decile groups had lower early infant height velocity that reversed by 24 months, higher weight velocity from 6 to 24 months, higher and earlier peak BMI, higher rebound BMI, and increased risk of overweight or obesity at age 10 years. Lower decile groups showed the opposite patterns. For example, mean differences for infant peak BMI were -0.38 (95% CI, -0.43 to -0.33) for the second decile birth weight group and 0.33 (95% CI, 0.29 to 0.38) for the ninth decile birth weight group compared with the fifth to sixth decile birth weight group. Mean differences for age at peak BMI were 0.22 months (95% CI, 0.12 to 0.33 months) for the second decile birth weight group and -0.21 months (95% CI, -0.30 to -0.11 months) for the ninth decile birth weight group compared with the fifth to sixth decile birth weight groups. Risk ratios for overweight or obesity at 10 years were 0.86 (95% CI, 0.76 to 0.97) for the second decile birth weight group and 1.25 (95% CI, 1.13 to 1.38) for the ninth decile birth weight group. Birth weight was not associated with age at rebound BMI. Replication analyses (2517 children; mean [SD] gestational age at birth, 39.2 [1.8] weeks; 1191 girls [47%]) supported these findings. Associations were typically linear and similar in boys and girls. Deciles provided only modest estimation gains over conventional categories. Conclusions and Relevance: In this cohort study of 38 535 singletons, birth weight decile was associated with early-life growth patterns. Birth weight decile group may help identify high-risk children missed by conventional thresholds, although the benefit of analysis using decile group over traditional groups remains modest.