Longitudinal studies of the relationship between birth order and birth weight suggest that mean birth weight increases with parity among women of all ages. In contrast, national birth statistics and other cross-sectional studies of these relationships suggests a decrease in mean birth weight and an increased incidence of low-weight births with parity among adolescent mothers. We examine the potential reasons for these strikingly different conclusions. Our aim is to determine whether multiparity itself is a low-birth-weight risk factor among adolescent mothers or whether it is a marker for other adolescent maternal conditions that increase the risk of a low birth weight, independent of parity.
PIP: The results of longitudinal studies of the relationship between birth weight and birth order among adolescent mothers are conflicting. Longitudinal studies concentrate on the outcomes of one individual mothers' successive pregnancies. These studies suggest that mean weight at birth goes up with parity. However, cross-sectional studies and national birth statistics show a decrease in mean birth weight and increasing occurrence of low birth weight with parity in teenage mothers. These differences are partly a reflection of differences in the analysis units (i.e. infants v. mothers). When the mother is analyzed, birth weight statistics from individual brothers and sisters are analyzed longitudinally. When the infant is analyzed, information is collected from different brother-sister relationships, pooled, and analyzed cross sectionally. "A statistically significant negative, linear relationship" comes out between mean birth weight and parity in women under 20 years of age. In Norway, birth registry information shows that mean birth weight increases "with parity within individual sibships" but decreased with the size of the sibship. The good relationship between infant size and parity may be confused by the negative relationship between infant size and sibship. This is especially true among teenagers. Women who have more than 1 baby in their teenage years tend to have more children overall than other women do. Multiple pregnancies during adolescence may show disadvantages rather than a cause of bad pregnancy outcome. In future studies, data should be analyzed both cross sectionally and longitudinally and adjusted for sibship size.