Archives ofDisease in Childhood 1991; 66: 584-587 Is low birth weight a risk factor for asthma during adolescence? D S Seidman, A Laor, R Gale, D K Stevenson, Y L Danon Department of Obstetrics and Gynecology, Sheba Medical Center, Tel-Hashomer 52621, Israel D S Seidman Medical Statistics Branch, Israeli Defence Force Medical Corps A Laor Department of Neonatology, Bikur Cholim Hospital, Jerusalem, Israel R Gale Department of Pediatrics, Stanford University School of Medicine, Stanford, California, USA DK Stevenson Division of Pediatric Immunology, Belinson Medical Center, Sackler School of Medicine, Tel-Aviv University, Israel Y L Danon Correspondence to: Dr Seidman. Accepted 13 December 1990 Abstract The effect of low birth weight on the inci- dence of asthma by 17 years of age was inves- tigated by studying medical draft examination records of 20 312 male subjects born in Jeru- salem between January 1967 and December 1971. Additional information on birth weight and other demographic factors was abstracted from the Jerusalem Perinatal Study comput- erised database. A stepwise multiple logistic regression was used to estimate the odds ratios for developing asthma by 17 years of age in 500 g birthweight categories from <2000 g to 4500 g. The odds ratios were adjusted for the confounding effects of ethnic origin, social class (determined by area of residence), paternal education, maternal age, and birth order. The group with low birth weights (<2500 g, n= 1004) had a significantly increased risk of developing asthma by 17 years of age, with an adjusted odds ratio of 1.44 (95% confidence interval (CI) 0-79 to 2.66) for birthweight group <2000 g and 1-49 (95% CI 1*05 to 2-12) for birthweight group 2000-2499 g compared with the reference group of 3000-3499 g. We conclude that infants with birth weights of <2500 g may have a higher risk of asthma during childhood and adolescence than infants who were heavier at birth. Advances in neonatal intensive care have greatly improved the survival of low birthweight (LBW) infants, but they still have a high risk of handicaps.' 2 Currently little is known about the long term respiratory outcome of LBW infants. Chan et al reported a close association between low birth weight and poor airway function,3 and troublesome cough at 7 years of age.4 The aim of this study was to evaluate the influence of low birth weight on the incidence of asthma during childhood and adolescence. The possible effects of social, ethnic, and environ- mental factors were taken into account. Subjects and methods The computerised records of the medical exami- nations of 20 312 male recruits to the Israeli army who were born in Jerusalem between 1964 and 1971 were reviewed to establish the inci- dence of asthma. Each subject had been asked specifically whether he had ever been diagnosed as having asthma and whether he had ever had recurrent wheezing, nocturnal coughing, or coughing or wheezing after exertion (I Auerbach, C Springer, S Godfrey, personal communication). All those with any symptoms that could have been indicative of past or pre- sent asthma were referred for a second examina- tion by a pulmonologist. This examination included a further detailed history, physical examination, and spirometry while at rest. An exercise test was done during the latter period of our study in all subjects except those who had overt signs of obstruction on spirometry. The specific information about lung function infor- mation is not available, however, as the com- puterised data included only the final diagnosis. Asthma was diagnosed only in cases with clear medical evidence of past or present bronc- hial disease, as opposed to episodic attacks. Birth weights recorded at the time of delivery were obtained from the computerised files of the Jerusalem Perinatal Study.' 6 Detailed demographic data were available from the com- puterised records of the military draft medical examination.7 The data for each individual were matched using a seven digit identification number. The completeness of the match was confirmed by comparing maternal identity numbers. A stepwise multiple logistic regression analy- sis was done with the Logist procedure of the SAS software to adjust for the effect of the stu- died independent variables.8 Asthma was used as the dependent variable. The independent variables were birth weight (in 500 g categories from <2000 g-4500 g), ethnic origin (according to paternal country of birth), area of residence (classified by municipal tax level),6 maternal age (as a continuous variable), paternal educational attainment (years of schooling) and birth order (categorised to avoid dubious assumptions about linearity). All variables that were signifi- cant (p<005) were added to a model that con- tained the birth weight variable at the first step. The results of the logistic regressions are pre- sented as adjusted odds ratios with their 95% confidence intervals (CI). The estimates pre- sented are based on the exclusion of cases with missing values (1 -9%). Results The overall incidence of asthma in our study population at 17 years of age was 3-7%. Subjects whose birth weight was between 2000-2499 g had a significantly higher incidence of asthma. Other factors that were associated with a signifi- cantly increased risk of asthma included Euro- pean or North American ethnic origin, being the first born, and having a well educated father (table 1). There was a significantly lower inci- 584 on June 11, 2020 by guest. Protected by copyright. http://adc.bmj.com/ Arch Dis Child: first published as 10.1136/adc.66.5.584 on 1 May 1991. Downloaded from