Through age 5, children born at full term with low birth weight show significantly greater risk for developing respiratory symptoms, including wheezing, coughing and pulmonary infections, according to a large longitudinal study on birth weight and development.
The children's symptoms grew worse if they were exposed to environmental tobacco smoke.
The research results appear in the second issue for May 2007 of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.
Professor Johan C. de Jongste, M.D., Ph.D., of the Department of Pediatric Respiratory Medicine at Erasmus MC/Sophia Children's Hospital in The Netherlands, and eight associates studied 3,628 children who took part in the 1996 Prevention and Incidence of Asthma and Mite Allergy (PIAMA) birth cohort study, which analyzed children's allergic reactions and history of asthma.
The researchers concluded that the effect of birth weight on respiratory symptoms increased, from ages 1 to 5, in direct relation to birth weight per kilogram less in each child. However, after age 5, the effect lessened and was not significant by age 7. The authors defined low birth weight as 5.5 pounds at birth.
"Size and maturity are major factors in the development of the lung," said Dr. de Jongste. "In children with diminished prenatal growth, and consequently low birth weight, a disturbed lung development is associated with a relatively small airway caliber. This can cause decreased lung function and more respiratory symptoms later in life."
According to the investigators, the effect of birth weight on respiratory symptoms was significantly greater among children exposed to tobacco smoke in their home. They also noted that maternal smoking during pregnancy was "clearly associated with a reduced birth weight."
By age 2, cough was the most frequently reported symptom among the children. Between the ages of 4 and 7, about 70 percent of the kids with respiratory symptoms reported cough.
During the full 7-year follow-up, 38.9 percent of the study population had at least one wheezing episode; 51.7 percent reported cough at night; and 37.3 percent a lower respiratory infection at a specific point in time.
"Overall, 70 percent of the cohort had reported at least one respiratory symptom at some point in the first 7 years of life," said Dr. de Jongste.
A child born at a low birth weight had an additional 6 percent chance of respiratory symptoms if he or she was exposed to environmental tobacco smoke at home after birth. The risk for children who are exposed both in utero and after birth rose to 12 percent.
"To separate the effect of a birth weight in children born at term from the sequelae of prematurity," the researchers excluded all premature infants from their study.
Drs. de Jongste and colleagues concluded that low birth weight is an important risk factor for the development and persistence of respiratory symptoms during preschool and early elementary school. The association was strongest at age 4 but had no effect after age 6.
"In addition, all parents should be strongly encouraged to stop smoking because it has clear health benefits for their offspring," said Dr. de Jongste. "Our data suggest that focusing on parents of low-birth-weight children is of specific interest because their children may be especially vulnerable to environmental tobacco smoke."