Objectives: It is widely believed that premature rupture of membranes accelerates fetal pulmonary maturity. The purpose of our study was to determine the duration of premature rupture of the membranes required to achieve this effect.
Study design: Retrospective analysis of our database yielded a group of 1395 patients who were delivered between 24 and 35 weeks' gestation and for whom we had complete data. The frequencies of premature rupture of the membranes and respiratory distress syndrome by each gestational week were analyzed with a log linear multiway contingency table analysis. Because gestational age was based on pediatric examination and was therefore somewhat subjective, birth weight was used to confirm results. Additional factors related to respiratory distress syndrome were considered in stepwise discriminant analysis. Results were further verified by the 1980 National Natality Survey data set.
Results: When we controlled for either gestational age or birth weight, there was no significant difference in the frequency of respiratory distress syndrome related to premature rupture of the membranes, but there was a suggestion (p < 0.08) that respiratory distress syndrome was actually more frequent after premature rupture of the membranes. Stepwise discriminant analysis revealed that gestational age, birth weight, race, sex, and Apgar score at 1 minute were all more important determinants than duration of premature rupture of the membranes. Duration of premature rupture of the membranes was associated with an increased risk of respiratory distress syndrome. Amnionitis was found to be highly related to the duration of premature rupture of the membranes. The incidence of amnionitis significantly increased 24 hours after premature rupture of the membranes occurred. A multiway frequency contingency table of the National Natality Survey data showed a significant increase in respiratory distress syndrome in association with premature rupture of the membranes.
Conclusions: Pulmonary maturation continues but is not accelerated after premature rupture of the membranes. In fact, there is a strong suggestion that premature rupture of the membranes actually increases the risk of respiratory distress syndrome at a given gestational age.