Cryopreservation of cleaving embryos and expanded blastocysts in the human: a comparative study.

The survival and implantation capacity of cryopreserved cleaving (5-cell to 10-cell) human embryos and expanded blastocysts was compared. Twice as many cleaving embryos were frozen as were expanding blastocysts because of the low developmental potential of human embryos in vitro. However, significantly more expanded blastocysts survived cryopreservation than cleaving embryos, and relatively more pregnancies were established by the replacement of thawed blastocysts than by the replacement of thawed cleaving embryos. Cleaving embryos from 26 women were thawed; 17 had thawed embryos replaced, and 4 subsequently became pregnant. Expanded blastocysts were thawed from 23 other women; 15 had thawed blastocysts replaced, and 8 subsequently became pregnant. The pregnancy of one patient in each group aborted; both patients were over 40 years of age. It is estimated that by maintaining the current policy of replacing three fresh embryos and freezing any remaining embryos when they reach blastocyst stage, the total incidence of pregnancy would increase by 3%.