Cryopreservation of human embryos at the pronucleate, early cleavage, or expanded blastocyst stages.

Supernumerary embryos following treatment by IVF or GIFT were cryopreserved at the pronucleate, early cleavage or expanded blastocyst stages. The success of embryo cryopreservation at these stages was evaluated in terms of (i) the proportion of embryos surviving the freeze/thaw procedure; (ii) the proportion of patients reaching embryo replacement; and (iii) the incidence of pregnancy per replacement. Significantly more embryos survived when frozen/thawed at the pronucleate (44/61; 72%) or early cleavage stages (48/80; 60%), than at the expanded blastocyst stage (13/34; 38%). A significantly higher proportion of patients had embryo replacements when embryos were frozen/thawed at the pronucleate (17/19; 89%) or early cleavage stages (21/24; 88%), than at the expanded blastocyst stage (9/17; 53%). Following replacement of frozen/thawed pronucleate and early cleavage stage embryos, clinical pregnancy rates of 8/17 (47%) and 3/21 (14%) clinical pregnancies were achieved, respectively. No pregnancies were achieved following replacement of frozen/thawed expanded blastocysts.