The use of intra-endometrial embryo transfer for increasing the pregnancy rate

It has been demonstrated previously that pregnancy can be achieved by the direct insertion of embryos into the endometrial stroma (intra-endometrial embryo transfer) of mice. In this study we evaluated whether intra-endometrial transfer resulted in a higher pregnancy rate than conventional embryo transfer. Mouse blastocysts (ICR strain), recovered on day 4 of pregnancy, were transferred into pseudopregnant day 2, day 3 and day 4 mice of the same strain ; 1-, 2- and 8-cell embryos were also transferred into pseudopregnant day 4 mice. In intra-endometrial embryo transfer, a 27 gauge injection needle was inserted near the utero-tubal junction into the endometrial stroma and then removed ; one blastocyst was transferred into each uterine horn with a glass micropipette. Conventional transfers were performed simultaneously as controls. The pregnancy rates and embryonic viability rates were evaluated 9 days after embryo transfer. Furthermore, the rates of live birth for intra-endometrial and conventional embryo transfers were compared when blastocysts were transferred into pseudopregnant day 4 uteri by both methods. In the transfer to pseudopregnant day 2 recipients, the pregnancy and embryonic viability rates were significantly higher (P < 0.01) in intra-endometrial [23.4 (11/47) versus 15.9% (15/94)] than in conventional embryo transfer [4.3 (2/46) versus 2.2% (2/92)]. In the transfer to pseudopregnant day 3 recipients, both rates were also higher (P < 0.01) in intra-endometrial [90.9 (40/44) versus 87.5% (77/88)] than in conventional transfer [67.4 (31/46) versus 64.1% (59/92)]. In synchronous transfer to pseudopregnant day 4 recipients, there was no difference between methods in the pregnancy rate [conventional, 48.9% (24/49) ; intra-endometrial, 50.9% (29/57)] and the embryonic viability rate [conventional, 44.9% (44/98) ; intra-endometrial, 43.0% (49/114)]. In the transfer of 1-, 2- and 8-cell embryos into pseudopregnant day 4 mice, pregnancy and embryonic viability rates were very low in both transfer methods. Intra-endometrial transfer produced normal living offspring at a similar rate to conventional transfer. These results reveal that intra-endometrial transfer increases pregnancy and embryonic viability rates in asynchronous embryo transfer in mice, especially when the duration of pseudopregnancy in the recipients was less than the age of the transferred embryos.