Letter by Degraeuwe and Been Regarding Article, “Evidence of Arteriolar Narrowing in Low-Birth-Weight Children”
To the Editor:
We were interested to read the article by Mitchell et al1 in which they describe an intriguing association of lower birth weight, shorter birth length, and smaller head circumference with narrower retinal arteriolar caliber in 6-year-old children. From these findings, the authors infer that poor in utero growth may adversely affect microvascular growth.
We disagree with the authors on the underlying assumption that low growth parameters at birth are specific and sensitive markers of fetal growth restriction. Indeed, one can estimate that up to 50% of low-birth-weight newborns are appropriately grown preterm infants. Birth weight, length, and head circumference are first and foremost a function of gestational age. Accordingly, it would be more appropriate to enter the length of gestation as a continuous variable in the regression analyses. In addition, birth weight is influenced by the ethnicity, parity, height, and booking (first visit or antenatal clinic appointment) weight of the mother and by the infant’s gender.2 Placental insufficiency, the major cause of fetal growth restriction, affects weight in preference to height or head circumference. This pattern is not reflected in the study data,1 arguing against a preponderant impact of in utero growth retardation on retinal arteriolar caliber.
Against this background, and to disentangle the effect of gestational age and fetal growth on arteriolar caliber, we would favor expressing birth weights, heights, and head circumferences as standard deviation scores of their respective means per week of gestation (Z scores) to estimate the relative position of each parameter for a given gestation. To take gestational age, ethnicity, sex, birth rank, maternal height, and pregravid weight into account, individualized fetal growth curves should be applied.2
Reanalysis of the data are relevant, as duration of gestation has been shown to be inversely associated with blood pressure in adulthood.3–5 We expect that the suggested methodology will more closely separate the contribution of slower intrauterine growth from the effect of shorter length of gestation on microvascular growth.
Dr Been is supported by a Profileringsfonds grant from the Maastricht University Hospital. Dr Degraeuwe reports no conflicts.
Mitchell P, Liew G, Rochtchina E, Wang JJ, Robaei D, Cheung N, Wong TY. Evidence of arteriolar narrowing in low-birth-weight children. Circulation. 2008; 118: 518–524.
Leon DA, Johansson M, Rasmussen F. Gestational age and growth rate of fetal mass are inversely associated with systolic blood pressure in young adults: an epidemiologic study of 165,136 Swedish men aged 18 years. Am J Epidemiol. 2000; 152: 597–604.
Johansson S, Iliadou A, Bergvall N, Tuvemo T, Norman M, Cnattingius S. Risk of high blood pressure among young men increases with the degree of immaturity at birth. Circulation. 2005; 112: 3430–3436.
Cooper R, Atherton K, Power C. Gestational age and risk factors for cardiovascular disease: evidence from the 1958 British birth cohort followed to mid-life. Int J Epidemiol. 2009; 38: 235–244.