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ROTTERDAM, The Netherlands -- Through age five, children born at full term but with a low birth weight were more likely to have respiratory symptoms, but not asthma, researchers here reported.
ROTTERDAM, The Netherlands, May 15 -- Through age five, children born at full term, but with a low birth weight, were more likely to have respiratory symptoms, but not asthma, found researchers here.
The early problems, including wheezing, coughing (not associated with a cold), and pulmonary infections, waned by age seven, reported Johan C. de Jongste, M.D., Ph.D., of Erasmus MC/Sophia Children's Hospital here and colleagues, in the May issue of the American Journal of Respiratory and Critical Care Medicine.
However, the respiratory problems were exacerbated by exposure to environmental tobacco smoke, they added.
In a prospective birth cohort study, 3,628 children with a gestational age of 37 weeks or more, weighing 5.5 pounds at birth, were monitored up to the age of seven. The children were enrolled in the prospective Prevention and Incidence of Asthma and Mite Allergy (PIAMA) birth cohort study.
Parental questionnaires were used to assess respiratory health yearly. Associations of birth weight with respiratory symptoms and a doctor's diagnosis of asthma were assessed in a repeated-event analysis.
The effect of birth weight on respiratory symptoms increased from ages one to five, in direct relation to each kilogram decrease in birth weight.
Lower birth weight was associated with more respiratory symptoms (odds ratio [OR] per kg decrease in birth weight, 1.21; 95% confidence interval [CI], 1.09-1.34), they reported.
Although the effect of birth weight increased from age one to five, it decreased after that and was no longer significant at age seven.
The effect of birth weight on respiratory symptoms was significantly greater among children exposed to tobacco smoke in their home than among nonexposed children (OR at five years 1.21 [CI, 1.02-1.44] and 1.52 [CI, 1.23-1.87], respectively).
Birth weight and a doctor's diagnosis of asthma were not related (OR, 1.06; 95% CI, 0.82-1.37), the researchers reported.
Wheezing at least once among 21% was the most frequently reported symptom in the first year of life, but showed a steep decline with increasing age. By age two, cough was the most common symptom, and from age four to seven, about 70% of the children with respiratory symptoms reported cough.
During the full seven years, 38.9% of the children had at least one wheezing episode, 51.7% reported cough at night, and 37.3% had a lower-respiratory infection (pneumonia, bronchitis, or pertussis) at any point.
The prevalence of doctor-diagnosed asthma showed a decline with increasing age from 5.8% in the first year to 2.7% at the age of seven. There was no significant association between birth weight and a diagnosis of current asthma over all ages, with a possible borderline significant association only at age five (OR, 1.43 per kg decrease, CI 0.99-2.07) the researchers said.
A child born at a low birth weight (5.5 lb) had an additional 6% chance of respiratory symptoms if exposed to environmental tobacco smoke at home after birth. For those exposed in utero and after birth, the risk mounted to 12%, the researchers found.
Size and maturity are major factors in the development of the lungs, Dr. de Jongste said. If prenatal growth is diminished, disturbed lung function may lead to a relatively small airway caliber, causing respiratory symptoms.
As the airways grow in absolute size, he said, such children may be less likely to develop symptoms, which might explain the transient nature of the increased risk. However, lung function may remain suboptimal, even after these children are asymptomatic, as was found in this study.
This is important, he said, because it has been speculated that these children may be at increased risk of respiratory morbidity when lung function decreases later in life.
One could also argue, he said, that disorders of fetal growth affect immune function, leading to a higher rate of respiratory symptoms.
Limitations of the study included parental reporting for respiratory symptoms, and the difficulty of proving that the interaction between smoking and low birth weight can be attributed to either uterine or postnatal exposure.
Also, it is possible, the researchers said, that smoking during pregnancy is an antecedent of low birth weight, rather than a confounder. Thus, adding prenatal smoking to these models may have caused some overadjustment and thus conservative estimates of the effect.
Dr. de Jongste and his colleagues concluded that "all parents should be strongly encouraged to stop smoking because it has clear health benefits for their offspring. Our data suggest that focusing on parents of low-birth-weight children is of specific interest because their children may be especially vulnerable to the effects of environmental tobacco smoke."
The PIAMA study is supported by The Netherlands Organization for Health Research and Development; The Netherlands Organization for Scientific Research; The Netherlands Asthma Fund; The Netherlands Ministry of Spatial Planning, Housing, and the Environment; and The Netherlands Ministry of Health, Welfare, and Sport.