First-born children have reduced insulin sensitivity and higher daytime blood pressure compared to later-born...

Evidence suggests that first-born children and adults are phenotypically different to later-born children. Therefore, we aimed to assess whether birth order would be associated with changes in metabolism in childhood.

We studied 85 healthy prepubertal children aged 4 to 11 years, born 38 to 40 weeks' gestation, and birth weight appropriate for gestational age: 32 first-born and 53 later-born children. Clinical assessments included measurement of children's height, weight, fasting lipid and hormonal profiles, and dual-energy x-ray absorptiometry-derived body composition. Children also underwent 24-hour ambulatory blood pressure monitoring, and frequently sampled intravenous glucose tests with Bergman's minimal model.

First-born children were approximately 3 cm taller (height SD scores 0.88 vs 0.39; P = .009) and were slimmer (body mass index SD scores -0.05 vs 0.39; P = .048) than later-born children. Consistent with their taller stature, first-born children also had a 27% increase in IGF-I concentrations (227 vs 173 ng/mL; P = .002). Insulin sensitivity was reduced by 21% among first-borns compared to later-borns (8.4 vs 10.6 × 10(-4)/min/[mU/L]; P = .019). Further, 24-hour ambulatory blood pressure monitoring showed that first-borns had higher daytime systolic (+5 mm Hg; P = .032) and diastolic (+4 mm Hg; P = .029) blood pressure. Blood lipids were unaffected by birth order.

Although first-borns were taller and slimmer, these children had reduced insulin sensitivity and increased daytime blood pressure compared to later-borns. Thus, first-borns may be at a greater risk of metabolic and cardiovascular diseases in adult life. This finding may have important public health implications, in light of a worldwide trend toward smaller families.