The increase of cytotoxic NK cells in the peripheral blood and the endometrium may affect the therapeutic results of IVF-ET. It was suggested that modifications of NK cytotoxicity or of NK subpopulations might contribute to the improvements of IVF outcomes.

Though the multiple pregnancy rate in in-vitro fertilization-embryo transfer must be reduced, strict and uniform regulation of the number of embryos transferred may make it impossible for women with little natural fecundity to carry children. We therefore restricted the number of embryos per transfer. In the first half of the observation period (n = 100), we limited the number of embryos transferred to three regardless of the number of previous transfers. In the second half (n = 137), we strictly regulated the number of embryos transferred to two at the first attempt and three in the second and later attempts. The multiple pregnancy rate per pregnancy was significantly lower (P < 0.005) in the second period (20.4%) than in the first period (52.9%), while the clinical pregnancy rate and the implantation rate per transfer were similar in both observation periods. 60.7% (17/28) of the multiple pregnancies arose from the first embryo transfer attempt, and 17.9% (5/28) of them arose from a second attempt. The 18 multiple pregnancies in the first period involved six sets of triplets, while the 10 multiple pregnancies that arose in the second period all involved twins. The implantation rate per transfer at the first attempt was significantly higher than that at the second or later attempts. The criterion for determining the number of embryos transferred should be simple to minimize errors of judgement. We believe that our method of restricting the numbers of embryos transferred may be one method of reducing multiple pregnancies without eliminating the possibility of having children for women with low fecundity.

It is suggested that seminal plasma participates in the recruitment of CD56(bright) NK cells into endometrium.

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