26 © Blackwell Publishing Ltd. Paediatric and Perinatal Epidemiology 2004, 18, 26–32 Blackwell Science, LtdOxford, UKPPEPaediatric and Perinatal Epidemiology1365-3016Blackwell Publishing Ltd, 200418 12632Original ArticleHead circumference in siblingsS. O. Samuelsen et al. Correspondence: Sven Ove Samuelsen, Division of Epidemiology, Norwegian Institute of Public Health, PO Box 4404 Nydalen, N-0403 Oslo, Norway. E-mail: osamuels@math.uio.no Association of head circumference at birth among sibling pairs Sven O. Samuelsen a,b , Lars C. Stene a and Leiv S. Bakketeig c a Division of Epidemiology, Norwegian Institute of Public Health, and b Department of Mathematics, University of Oslo, Oslo, Norway, and c Research Unit for Epidemiology, Institute of Public Health, University of Southern Denmark – Odense University, Odense, Denmark Summary The objective of the study was to estimate and compare the correlation coefficients of head circumference and weight at birth among sibling pairs. Pairs of singleton siblings were ascertained among children born in Norway to the same mother between 1978 and 1997. Head circumference, birthweight and other perinatal factors were registered in the Medical Birth Registry of Norway. Head circumference measurements were obtained for first- and second-born in 287 448 sibling pairs. The correlation coefficient of head circumference among first- and second-born siblings was 0.343 (standard error 0.002) compared with 0.477 (standard error 0.002) for birthweight. These results were similar for later-born sibling pairs and only slightly influenced by other factors such as sex, fatherhood and time between pregnancies. Sensitivity analyses showed that substantial measurement error is required to explain the lower correlation for head circumference. In conclusion, the sibling correlation of head circumference was con- sistently weaker than that of birthweight. Introduction Head circumference is routinely measured at birth along with birthweight and crown-heel length in many countries. Recent work has shown that size at birth is associated with a number of health outcomes in child- hood, adolescence and adulthood. 1 Although head cir- cumference is relatively strongly associated with both birthweight and birth length, it is thought particularly to reflect brain size. 2 Several investigators have stud- ied the relationship between head circumference or weight at birth or in early life and different aspects of mental performance, neurological or psychiatric out- comes in childhood or adulthood 3–9 as well as with other health outcomes such as atopic disorders. 10,11 One possible partial explanation of these associa- tions between head circumference or other aspects of size at birth and different health outcomes in later life may be common genetic factors influencing both size at birth and the subsequent health outcome. Twin studies and other family-based studies have strongly indicated that genetic factors influence birthweight and gestational age. 12–17 Sibling associations have pre- viously been demonstrated for gestational age, birth- weight or low birthweight for gestational age, 13 and for a combination of small-for-gestational age and ponderal index. 18 To the best of our knowledge, the association of head circumference among siblings has previously not been estimated in population-based studies. Materials and methods The analyses are based on data from all births in Nor- way between 1978 and 1997, registered in the Medical Birth Registry of Norway. 19 Our main variable head circumference has been recorded from 1978 and the analyses were thus confined to live-born singleton births from 1978 to 1997 for which both first and sec- ond child of the same mother were recorded in the registry. This consists of 295 230 sibling pairs. Head circumference was recorded on 98.3% of the first-born and 99.0% of the second-born children. The analyses were then restricted to the 287 448 sibling pairs for which head circumference was recorded for both chil- dren. We also included 93 885 third and 17 204 fourth live-born, singleton children of the same mother. Data on all four children were obtained for 16 377 families. Variables Head circumference was measured to the nearest cen- timetre with a tape measure soon after birth. Gesta-