The practice of elective single embryo transfer (eSET) in preimplantation genetic diagnosis (PGD) cycles is generally not followed due to the belief that embryo biopsy could affect its post-thaw survival rate. Now, a recent study has reported that eSET following PGD and freezing at blastocyst stage results in implantation and pregnancy rates similar to non-PGD cycles, thereby reducing the chances of multiple pregnancies. The results of the study were published online in the recent issue of Human Reproduction.
Tarek El-Toukhy, from the Guy’s and St Thomas’ Hospital NHS Foundation Trust, London, and colleagues, conducted the study to assess the survival and pregnancy rates of PGD embryos biopsied and cryopreserved at cleavage and blastocyst stage, respectively, in addition to its effect on the overall success of elective SET. The researchers offered SET for all the couples (January 2006 to July 2008), with two or more transferable blastocysts biopsied for PGD on day 3, and the surplus embryos cryopreserved on day 5 of the embryonic culture. The results of 32 frozen-thawed PGD cycles were compared with 191 frozen-thawed conventional IVF/ICSI cycles without embryo biopsy (controls). The results of all fresh PGD cycles that were performed pre- and post-January 2006 were also compared in the study.
The investigators observed that the survival rates of frozen-thawed blastocysts were comparable in the PGD and IVF/ICSI groups (87% vs. 88%; P=0.94). They also observed that the rates of clinical pregnancy (34% vs. 36%; P=0.77) and implantation (35% vs. 29%; P=0.45) did not vary significantly between the PGD and IVF/ICSI groups, respectively. During the same period, a reduction in multiple pregnancy rates from 36% to 10% (OR=0.20; P<0.001), was observed in fresh PGD cycles without any decrease in the pregnancy rates. Based on the findings, the researchers proposed that the practice of eSET and cryopreservation of surplus embryos in the future could also include PGD for inherited genetic defects.
Earlier, Escribá et al (Fertility and Sterility, 2008) conducted a retrospective study to evaluate the survival rate and clinical results of the vitrification technique on PGD blastocysts and determine its effect on the reproductive outcome per cycle. The survival rates of the co-cultured vitrified PGD blastocysts were comparable to that of non-PGD blastocysts (49% vs. 42%, respectively).
Following blastocyst transfer in 64% of the patients, the researchers did not observe statistically significant difference in the rates of pregnancy (44% vs. 37%), implantation (40% vs. 27%) and ongoing pregnancy (32% vs. 37%) between the PGD and non-PGD blastocysts, respectively. The cumulative ongoing pregnancy rate was significantly improved by the vitrification of both PGD and non-PGD blastocysts from 47% to 53% and from 45% to 53%, respectively. The study findings suggested that the PGD blastocyst vitrification had comparable survival rates and cumulative ongoing pregnancy rates with that of non-PGD blastocyst vitrification. The researchers also suggested that the vitrification of biopsied PGD embryos at a more advanced stage, compared to the cleavage stage, could be an important approach in PGD programs.
The ‘Best practice guidelines for clinical PGD and preimplantation genetic screening (PGS)’ published by the ESHRE PGD Consortium (Human Reproduction, 2005) recommended that all the couples with high genetic risk referred to PGD should be counseled by a clinical geneticist or a genetic counselor on the diagnostic procedure. The Consortium strongly recommends testing for embryo and blastomere identity throughout the procedure. Factors such as embryo quality and other medical risks were also suggested to be considered for determining the number of embryos to be transferred.
Satisfactory survival rates of cryopreserved blastocysts and high pregnancy rates have been achieved after the transfer of a single frozen-thawed blastocyst in conventional IVF/ICSI cycles. However, there is lack of sufficient evidence for the same in PGD cycles. With the current study demonstrating comparable outcomes with both PGD and non-PGD embryos, widespread adoption of elective SET and cryopreservation of surplus blastocysts in PGD cycles could aid in reducing multiple gestations and its associated complications.
References
1. El-Toukhy T, Kamal A, Wharf E, et al. Reduction of the multiple pregnancy rate in a preimplantation genetic diagnosis programme after introduction of single blastocyst transfer and cryopreservation of blastocysts biopsied on Day 3. Hum Reprod. 2009 Jun 30. [Epub ahead of print]
2. Research shows it is possible to freeze embryos and reduce multiple pregnancies in PGD cycles without adversely affecting pregnancy rates. Press Release. ESHRE. Last accessed July 21, 2009.
3. Escribá MJ, Zulategui JF, Galán A, Mercader A, Remohí J, de los Santos MJ. Vitrification of preimplantation genetically diagnosed human blastocysts and its contribution to the cumulative ongoing pregnancy rate per cycle by using a closed device. Fertil Steril. 2008 Apr;89(4):840-6.
