Birth Order Progressively Affects Childhood Height

Tim Savage; José G. B. Derraik; Harriet L. Miles; Fran Mouat; Wayne S. Cutfield; Paul L. Hofman

Disclosures

Clin Endocrinol. 2013;79(3):379-385. 

In This Article

Abstract and Introduction

Abstract

Background There is evidence suggesting that first-born children and adults are anthropometrically different to later-borns. Thus, we aimed to assess whether birth order was associated with changes in growth and metabolism in childhood.

Methods We studied 312 healthy prepubertal children: 157 first-borns and 155 later-borns. Children were aged 3–10 years, born 37–41 weeks gestation, and of birth weight appropriate-for-gestational-age. Clinical assessments included measurement of children's height, weight, fasting lipid and hormonal profiles and DEXA-derived body composition.

Results First-borns were taller than later-borns (P < 0·0001), even when adjusted for parents' heights (0·31 vs 0·03 SDS;P = 0·001). There was an incremental height decrease with increasing birth order, so that first-borns were taller than second-borns (P < 0·001), who were in turn taller than third-borns (P = 0·007). Further, among sibling pairs both height SDS (P = 0·009) and adjusted height SDS (P < 0·0001) were lower in second- vs first-born children. Consistent with differences in stature, first- (P = 0·043) and second-borns (P = 0·003) had higher IGF-I concentrations than third-borns. Both first- (P < 0·001) and second-borns (P = 0·004) also had reduced abdominal adiposity (lower android fat to gynoid fat ratio) when compared with third-borns. Other parameters of adiposity and blood lipids were unaffected by birth order.

Conclusions First-borns were taller than later-born children, with an incremental height reduction from first to third birth order. These differences were present after correction for genetic height, and associated to some extent with alterations in plasma IGF-I. Our findings strengthen the evidence that birth order is associated with phenotypic changes in childhood.

Introduction

There is a trend towards smaller families with an increasing number of couples having fewer children. As a result, there has been a steady increase in the population of first-born children relative to later-borns, and first-borns now represent over 60% of all births in the developed world.M[1,2] The trend towards smaller families began in the mid 20th century, but over the past two decades more couples are choosing to have only one or two children.[2] Fertility rates have consequently fallen to a mean of 1·56 live births per woman in Europe, reflecting the trend towards smaller families as well as an overall reduction in birth rates.[1] Changes in family size have occurred for a number of reasons, such as increased availability of contraception, economic pressures and advanced maternal age at first childbirth.[2] This is notwithstanding the impact of official national policies, such as the "one child" policy in China.

There is evidence that birth order affects offspring phenotype. Taller stature in first- compared with later-borns has been previously described in children,[3–5] adolescents[6] and adults.[7–10] However, all of these studies were conducted in populations including subjects from a broad range of socio-economic groups[3–5] and/or children's heights were not adjusted for parental height3. Nonetheless, birth order is known to affect offspring disease risk. First-born adults are at higher risk of obesity than later-borns.[6,8] Compared with later-borns, first-born children were also shown to have reduced insulin sensitivity,[11] higher blood pressure[11,12] and a greater risk of developing type 1 diabetes.[13] One adult study has described a less favourable lipid profile in first-borns,[10] but there are no data on the potential effects of birth order on blood lipids in childhood. Thus, we aimed to determine whether first-born children would have different height, body composition, metabolism and hormonal profiles compared with later-born children.

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