Birth order, birth weight and later patterns of growth

Objectives. The determinants of a child's pattern of growth are varied and include genetic, intrauterine, medical and environmental factors. The ultimate height and weight are the result of the interaction between many factors. The study utilises the presence of large numbers of multiparous and grand multiparous mothers within a primarily lower socio-economic status (SES), working class neighbourhood in a Western country, in order to determine how a child's birth order, a prenatal factor, affects both his birth weight as well as later patterns of growth. The influence of birth order on later growth has not previously been sufficiently examined due to the paucity of very large families in Western society. Design. The analysis was carried out on growth measurements from children visiting three neighbourhood clinics for routine medical care, one in a lower SES neighbourhood and two in middle to upper-middle class areas used as a control group matched for age, gender and ethnic background, in a metropolitan urban setting. Other pertinent data related to growth and nutrition was recorded on questionnaires and also analysed. Patients and setting. A total of 626 children, age 3-15 years (average age: 8.1 ± 2.9 years) were included in the study, which was conducted during a 3-month period during 1999 at local clinics. Results. The mean birth weight of children born to multiparous women within the lower SES neighbourhood was significantly greater than for single births or for small families within the same community (p = 0.034, d.f. = 1). There was a linear relationship between birth order and birth weight within the same SES group, based on linearity test analysis of variance (p = 0.049, d.f. = 1). When the lower SES, working class group with large families was compared with the higher SES control group, composed of smaller families, no significant differences were found in birth weight (t-test, p = 0.964), presumably because of the equalizing positive influences of later birth order in the lower SES, and better nutrition and medical care in the higher SES. When children's growth later in life, from the age of 6 years onward, was compared on the basis of their SES status, there was a significant disadvantage for later height coming from a lower SES background, regardless of what the birth weight had been. Implications for practice. Higher birth order in large, often poor families tends to confer an advantage in birth weight when compared with first born siblings from the same mother, and with smaller families within the same community. However this early size advantage at birth is lost over early childhood, when compared with their compatriots belonging to higher SES groups, whose birth weights were equal to or better to begin with. High birth weight for select individuals in low-income groups should not lead to the erroneous conclusion that optimal later growth and development are assured. Continued optimal nutrition and medical care are vitally important to perpetuate early prenatal advantage. Their relative influence should be examined in larger, better-controlled studies.