Birth Weight for Gestational Age and the Risk of Asthma in Childhood and Adolescence: A Retrospective Cohort Study

Abstract

To evaluate the possible correlation between birth weight for gestational age and asthma in childhood and adolescents.This was a cohort of all children of a term (≥37 weeks), singleton gestation born in the Canadian province Nova Scotia between January 1, 1989, and December 31, 2014.The retrospective cohort was created through the Nova Scotia Atlee Perinatal Database and with provincial administrative health data. Linkage was conducted by Health Data Nova Scotia on the basis of the health card number. Infants were categorized as small for gestational age (SGA; < 10th percentile), large for gestational age (LGA; >90th percentile), or appropriate for gestational age (10th–90th percentile). Birth weight z scores were calculated by using sex- and gestational age–specific means and SDs from the same reference population. Confounders included the following: maternal age, area of residence (urban versus rural), area-level income quintile at birth, parity, prepregnancy weight status (normal, overweight, or obese), asthma during pregnancy, mode of delivery (vaginal delivery versus cesarean delivery), and smoking in pregnancy (yes or no). Unadjusted and adjusted Cox proportional hazards regression with robust SEs was used to estimate the association between birth weight for gestational age and asthma. Associations between birth weight for gestational age and smoking in pregnancy were also examined.The final sample included 40 727 children. Of those, 10 155 children (23.6%) had asthma by 6 years of life, and 12 997 children (30.2%) had asthma during the first 18 years of life. LGA was not associated with the risk of developing asthma. There was an additive interaction between SGA and smoking. In mothers who smoked, there was an increased risk for asthma in children with low birth weight. However, in mothers who do not smoke, there was no increased risk of asthma across the birth weight spectrum among children.Term infants who are SGA are not at increased risk for asthma in the absence of smoking during pregnancy. There is also no association between LGA and the development of asthma.Infants who are low birth weight are believed to be at increased risk for asthma in childhood, but multiple factors may influence this risk. This cohort has an impressive number of infants managed in a universal health care system, in which asthma diagnosis sensitivity is 89% and specificity is 72% for this population, based on physician diagnosis, not parent recall. This study is unique in that with its regression models it was able to clearly demonstrate the role smoking has on the development of asthma. Prenatal smoking has a significantly stronger negative influence on developing asthma than early postnatal exposures. SGA infants born to smoking mothers are at a significantly increased risk for asthma. It was also interesting that although LGA infants are at risk later for obesity (a risk factor for asthma), they have no increase in asthma. Prevention of childhood asthma is essential to decrease future morbidity and possible mortality. The more risk factors we understand, the better counseling we can provide mothers early in pregnancy. Do not smoke!