- Randomization envelopes will be prepared by means of random number tables. Randomization will occur between 16 weeks and 20 weeks of gestation whereby the next numbered opaque envelope will be opened to assign each consenting patient to receive identically appearing active 17-alpha Hydroxyprogesterone Caproate (Proluton® Depot, Schering AG, Germany) or placebo (castor oil) injections prepared by our pharmacy.