The objective of our study is to compare the effectiveness of weekly intramuscular injections of 17-alpha Hydroxyprogesterone Caproate, a natural metabolite of progesterone, in preventing delivery at less than 37 weeks of gestation in a population of 290 patients with twin gestations between 16 and 36 weeks of gestation compared to a placebo. The data generated will be invaluable in managing this group of patients that is considered at a very high risk for preterm labor and delivery.