Additional Study Information: The purpose of this study is to test the effect of a drug called retosiban on stopping preterm labor and delaying the birth of her baby. We will also study the safety of the drug to find out what effects; good or bad, retosiban has on women in preterm labor and their fetuses. A previous small study, involving 93 women with preterm labor, found that retosiban increased the time to delivery and had a favourable safety profile. Retosiban is undergoing additional testing but has not yet been approved for the treatment of preterm labor.