[Cryopreservation of human oocytes in the pronuclear stage with an automated "open system"]

In the period from June 1990 to February 1993, 718 excess pronucleated oocytes from 185 cycles were cryopreserved by using the "open freezing system" in our IVF-programme at the Department of Gynaecology and Hospital of the University of Bonn. 1.5 mol propanediol and 0.1 mol sucrose were used as a cryoprotectant. During this time 287, pronuclear oocytes were thawed; of these 245 survived the freezing process and were classified as intact. The survival rate in this study was 85.4%, though if only one intact pronuclear oocyte was transferred, no pregnancy could be achieved, whereas if two or three pronuclear oocytes were transferred, seven to ten pregnancies resulted, respectively (23.3% and 23.8% pregnancy rates). Eight healthy children have been born to date from four other ongoing pregnancies, one of them is a twin pregnancy.