That is, exposure to ETS increases the risk that the infant will weigh less than 2,500 g and, therefore, will have a higher perinatal mortality. A few studies have examined possible relationships between chronic exposure to ETS by children and parameters of growth and development. These studies, however, did not differentiate between smoking during pregnancy and subsequent exposures during infancy and preschool years. After adjusting the data for social class and other social factors, they found that reduced height was associated with increasing numbers of cigarettes smoked in the home, regardless of whether the mother smoked during pregnancy and regardless of which parent smoked. To verify this small change in height, other studies of comparable magnitude are needed.
Growth is an especially difficult phenomenon to study.
Many factors, such as genetics, nutrition, social class, and ethnicity play important roles, and it is difficult to assign proportionate causality to each factor. Recall bias in the mothers of school-age children regarding their smoking habits during the pregnancy may produce unreliable results, especially in light of the increasing publicity regarding ill effects on the fetus of maternal smoking during pregnancy. Moreover, height and weight ratios and other growth measures are not reliably obtained in standard pediatric surveys. Chronic ear infections or effusions in young children can lead to hearing loss and consequent speech pathology. They found that the daily exposure to ETS was greater among cases.