Smaller pelvic size in pregnant adolescents contributes to lower birth weight.

Adolescent pregnancy is associated with low birth weight. This has been explained by socioeconomic or emotional factors. However, an adolescent's pelvis may not be completely developed and this can contribute to impairing fetal growth. Our aim was to compare the relationship between pelvic size and birth weight among adolescents and adult mothers. A cross-sectional study was carried out at Instituto de Medicina Infantil Professor Fernando Figueira (IMIP), Brazil. We studied 125 healthy adolescents and 207 healthy adult women, all of whom were primiparous with a singleton term and low-risk pregnancy. The conjugate, intercristal and interspinous diameters were assessed by the Collins pelvimeter. The effect of pelvic size on the birth weight was evaluated using principal component analysis and multiple linear regression model. The mean pelvic size was smaller in adolescent mothers compared to adult ones (35.1 cm vs. 37.5 cm; p<0.001; t-test). After adjusting for other confounding variables, the predicted birth weights corresponding to these mean values of pelvic size were: 3020±27 g for adolescent mothers and 3145±26 g for adult mothers and showed a significant difference (p<0.001). We concluded that a pelvis that is less than fully developed in adolescents, as assessed by pelvic size, may contribute to lower birth weight in adolescent mothers.