Prospective, randomized study to evaluate the success rates using HCG, vaginal progesterone or a combination of both for luteal support. - Ludwig,M.; Finas,A.; Katalinic,A.; Strik,D.; Kowalcek,I.; Schwartz,P.; Felberbaum,R.; Kupker,W.; Schopper,B.; Al-Hasani,S.; Diedrich,K. (Acta Obstet Gynaecol Scand, 2001)