Abstract
BackgroundBirth order has been proposed as a cardiovascular risk factor, because the lower birth weight and greater infant weight gain typical of firstborns could programme metabolism detrimentally.MethodsWe examined the associations of birth order (firstborn or laterborn) with birth weight-for-gestational age, length/height and body mass index (BMI) z-scores during infancy, childhood, and puberty using generalized estimating equations, with age at pubertal onset using interval-censored regression and with age-, sex- and height-standardized blood pressure, height and BMI z-scores at 13 years using linear regression in a population-representative Chinese birth cohort: “Children of 1997” (n = 8,327).ResultsCompared with laterborns, firstborns had lower birth weight-for-gestational age (mean difference = -0.18 z-score, 95% confidence interval (CI) -0.23, -0.14), lower infant BMI (-0.09 z-score, 95% CI -0.14, -0.04), greater childhood height (0.10 z-score, 95% CI 0.05, 0.14) and BMI (0.08 z-score, 95% CI 0.03, 0.14), but not greater pubertal BMI (0.05 z-score, 95% CI -0.02, 0.11), adjusted for sex, parental age, birthplace, education and income. Firstborns had earlier onset of pubic hair (time ratio = 0.988, 95% CI 0.980, 0.996), but not breast or genitalia, development. Firstborns had greater BMI (0.07 z-score, 95% CI 0.002, 0.15), but not height (0.05 z-score, 95% CI -0.01, 0.11), at 13 years, but similar blood pressure.ConclusionsDifferences by birth order continue into early adolescence with firstborns being heavier with earlier pubic hair development, which could indicate long-term cardiovascular risk.
Highlights
Differences between groups in cardiovascular disease (CVD) rates at the same level of wellestablished adult risk factors suggest etiologic gaps, which could be exploited to develop more focused and effective interventions.[1]
Firstborns had lower birth weight-for-gestational age (mean difference = -0.18 z-score, 95% confidence interval (CI) -0.23, -0.14), lower infant body mass index (BMI) (-0.09 z-score, 95% confidence intervals (CI) -0.14, -0.04), greater childhood height (0.10 z-score, 95% CI 0.05, 0.14) and BMI (0.08 z-score, 95% CI 0.03, 0.14), but not greater pubertal BMI (0.05 z-score, 95% CI -0.02, 0.11), adjusted for sex, parental age, birthplace, education and income
Firstborns had greater BMI (0.07 z-score, 95% CI 0.002, 0.15), but not height (0.05 z-score, 95% CI -0.01, 0.11), at 13 years, but similar blood pressure
Summary
Differences between groups in cardiovascular disease (CVD) rates at the same level of wellestablished adult risk factors suggest etiologic gaps, which could be exploited to develop more focused and effective interventions.[1]. Firstborns are different from laterborns in having lower birth weight,[4–6] possibly related to physiological changes in maternal uterine arteries during the first pregnancy facilitating nutrient flow to laterborns,[7] and faster infant growth.[8, 9] They tend to be taller and/or heavier in early childhood in many settings, including Brazil,[10] the Philippines, [11] Taiwan,[12] New Zealand[13] and Poland,[14] null associations have been observed in Japan,[15] Denmark[16] and the UK.[17]. They have been observed to have earlier pubertal onset in a Brazilian cohort.[10] These differences and all subsequent health effects may be the consequences of fetal and infant growth programming metabolism for life,[22, 23] or of childhood growth which may play a larger role in CVD risk factor.[24, 25]. Birth order has been proposed as a cardiovascular risk factor, because the lower birth weight and greater infant weight gain typical of firstborns could programme metabolism detrimentally
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