Pregnancy rates after transfer of embryos obtained from different stimulation protocols and frozen at either pronucleate or multicellular stages

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Abstract
After in-vitro fertilization, 2161 supernumerary embryos were frozen with 1 ,2-propanediol and sucrose as cryoprotectants at either pronucleate or multicellular (2–6 blastomeres) stages. By the end of March 1990, 494 pronucleate stage embryos and 492 multicellular stage embryos had been thawed and 54 and 47% of them, respectively were considered suitable for transfer. Ongoing pregnancy and implantation rates were 17.9 and 10.7%, respectively for embryos frozen at the pronucteate stage and 5.5 and 4.7% for embryos frozen at the multicellular stage. Ovarian stimulation with human menopausal gonadotrophin (HMG) after pharmacological hypophysectomy with a gonadotrophin releasing hormone agonistic analogue (GnRHa) using a long protocol permitted us to freeze significantly more embryos per cycle (7.2 ± 4.1) than stimulation with 11MG and GnRHa in a short protocol (4.7 ± 3.4) or stimulation with clomiphene citrate (CC) and HMG (2.7 ± 1.9). Ongoing pregnancy rates after transfer during the stimulated cycles were similar for the three types of treatment (27.1, 27.3 and 32.1%, respectively). However, ongoing pregnancy rates after frozen - thawed embryo transfers were significantly higher when originating from GnRHa + HMG treatments (14.3 and 14.8%, respectively for long and short protocols) than when originating from CC + HMG treatment (5.6%). Embryo cryopreservation has permitted the ongoing pregnancy rate to increase from 28.4 to 36.9% (P <0.01) even though more than half of the embryos have not been thawed. We conclude that embryos obtained after stimulation with GnRHa + HMG and frozen at the pronucleate stage are more likely to result in a pregnancy.

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