OBJECTIVE. The goal was to examine the associations of weight-for-length at birth and at 6 months with obesity at 3 years of age.

METHODS. We studied 559 children in Project Viva, an ongoing, prospective, cohort study of pregnant women and their children. We measured length and weight at birth, 6 months, and 3 years. Our main exposures were weight-for-length z score at birth adjusted for gestational age and weight-for-length z score at 6 months adjusted for weight-for-length z score at birth. We used multivariate regression analyses to predict the independent effects of birth weight-for-length z score and, separately, 6-month weight-for-length z score on BMI z score, the sum of subscapular and triceps skinfold thicknesses, and obesity (BMI for age and gender of ≥95th percentile) at age 3.

RESULTS. Mean weights at birth, 6 months, and 3 years were 3.55, 8.15, and 15.67 kg, respectively. Corresponding lengths were 49.9, 66.9, and 97.4 cm. At 3 years, 48 children (9%) were obese. After adjustment for confounding variables and birth weight-for-length z score, each increment in 6-month weight-for-length z score was associated with higher BMI z scores, higher sums of subscapular and triceps skinfold thicknesses, and increased odds of obesity at age 3. The predicted obesity prevalence among children in the highest quartiles of both birth and 6-month weight-for-length z scores was 40%, compared with 1% for children in the lowest quartiles of both. Whereas birth weight-for-length z scores were associated with higher BMI z scores, the magnitude of effect was smaller than that of weight-for-length z scores at 6 months.

CONCLUSIONS. More-rapid increases in weight for length in the first 6 months of life were associated with sharply increased risk of obesity at 3 years of age. Changes in weight status in infancy may influence risk of later obesity more than weight status at birth.

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