ObjectiveTo determine the marginal cost-effectiveness of two strategies for preventing respiratory distress syndrome (RDS) resulting from preterm birth: 1) tocolysis with beta-mimetic agonists and treatment with corticosteroids (TREATALL), and 2) amniocentesis and testing for fetal lung maturity, with treatment based on test results (TESTALL), compared with no treatment.MethodsWe used a Markov decision analytic model to estimate the outcomes of each strategy, from a hospitalbased perspective. Probability variables were obtained from the literature, whereas cost variables came from the Beth Israel-Deaconess Medical Center. Sensitivity analysis was performed on all variables.ResultsThe most cost-effective strategy varied with the probability of RDS. TREATALL was the most cost-effective strategy above a probability of 17% (before 34 weeks' gestation), TESTALL was most cost-effective from 17% to 2% (34–36 weeks), and it was most cost-effective to use no treatment at probabilities less than 2% (after 36 weeks). TREATALL and TESTALL were both cost-saving compared with no treatment at probabilities of RDS above 2%. TREATALL was more highly favored as the costs of RDS and preterm birth increased, whereas TESTALL was more favored as the specificity of the test and the cost of maternal hospitalization increased.ConclusionAlthough testing for fetal lung maturity is useful in many clinical situations, the cost-effectiveness of such testing in the setting of idiopathic preterm labor from a tertiary medical center perspective depends primarily on the probability and costs of RDS and the costs of non-RDS-related morbidity. At our institution, such testing is costeffective between 34 and 36 weeks' gestation.