Higher ongoing pregnancy rates in blastocyst transfer of frozen-thawed embryos in natural cycles than in hormone replacement therapy cycles

Abstract

The aim of this study was to compare the outcome obtained from frozen-thawed embryo transfer in women submitted to two different protocols: natural cycle vs. HRT cycles. Retrospective study. This study comprised cryopreserved-thawed embryo transfer cycles performed at IVI Valencia from January 2000 to January 2006. Data were grouped in: Group I (n = 423) had frozen-thawed embryo transfer in a natural cycle and Group II (n = 707) frozen-thawed embryo transfer in a HRT cycle. The inclusion criteria were: (1) absence of uterine pathology, recurrent miscarriage and implantation failure; (2) non-severe sperm pathology; (3) endometrial thickness >6 mm at the day of the start of progesterone administration. The implantation, pregnancy and abortion rates of cryopreserved-thawed embryo were compared between the groups. We also analyzed these parameters depending on the day of the embryo transfer. Statistical analysis was performed by X2- test for categorical comparisons and Student's t-test to compare means between groups. A P value < 0.05 was considered significant. No differences were found in implantation and pregnancy rates between cryopreserved-thawed embryo in natural vs. HRT cycles (24%, IC 95% 20.3–28.3% vs. 22%, IC 95% 18.6%–26.1%) respectively. Pregnancy rate was 37.1%, IC 95% 32.5–41.7% vs. 39.7%, IC 95% 36.1–43.3) respectively. To evaluate if the stage of the embryo development may have some impact in the outcome, we compared 730 women (293 natural cycles vs. 437 HRT cycles) that were transferred at day 3 with 400 women (130 natural cycles vs. 270 HRT cycles) that were transferred at blastocyst stage. Pregnancy rate for cryopreserved embryo was 40% (IC 95% 34.4–45.6%) in a natural cycle and 45% (IC 95% 40.3–49.7%) in a HRT cycle. If cryopreserved blastocyst were transferred, pregnancy rate was 30% (IC 95% 22.1–37.9%) in a natural cycle and 38.5% (IC 95% 32.1–43.7%) in a HRT cycle, and no significant difference was found between both groups. However, abortion rate was 24% (39/157, IC 95% 17.3–30.7%) in women who had embryo transfer in a natural cycle and 39.4% (110/281; IC 95% 33.7–45.1%) in HRT. There were significant differences between the two groups with regard to the abortion rate (P<0.002). These findings suggest that both cryopreserved-thawed embryo transfer protocols are equally effective in terms of implantation and pregnancy rates. However, this is the first study that shows a significant difference regarding the abortion rate.