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Quality is clearly deficient in many human embryos growing in vitro. Fewer than 20% implant after transfer to the uterus in the majority of patients, so two or more are replaced which results in high-order multiple pregnancies. Among many approaches to improving quality, co-culture with supporting cells has claimed to be successful (e.g. Bongso et al., 1994), although it is now being replaced by the use of sequential culture media (Ménézo et al., 1998). Zona drilling was also introduced to enhance implantation rates (Cohen et al., 1990). In recent studies, co-cultures combined with assisted hatching failed to improve implantation rates above 23–26% per embryo, which were overall similar to controls (Hu et al., 1998). Perhaps variations between clinics account for differences in the success of these approaches. Growth and transfer of blastocysts, an approach widely used in domestic and laboratory animal species, is now being recommended by several authors, e.g.tially, high implantation rates per embryo seem to be possible, allowing the transfer of only one or two embryos thereby avoiding multiple pregnancies. Blastocyst transfer is claimed to be more physiological than pronucleate and 2-cell transfers. This principle led to the first-ever clinical human pregnancy being established by blastocyst transfer (Steptoe and Edwards, 1978). Blastocysts have been grown in vitro for many years, with rates approaching 50% of embryos cultured being achieved Ͼ10 years ago, although many failed to expand fully (Fishel and Edwards, 1983). Expanded and hatching blastocysts were also cryopreserved, ready for a later transfer (Cohen et al., 1985, Fehilly et al., 1985). In some clinics, modern media containing various additives such as hyaluronate and vitamins still yield low rates of development of cleaving human embryos to expanding or hatched blastocysts. One detailed study (Jones et al., 1998) utilized a cell-and serum-free system of sequential culture media, brief exposures of oocytes to spermatozoa, culture of 2–3 embryos in one microdrop from days 1–3, then transfer to another medium from days 3–5. Gardner's (1994) G1 and G2 media with added albumin were used in this study, in various protocols, and blastocysts required various intervals in culture, i. 1 20% of embryos developed into enlarging blastocysts. Desai et al. (1997) reported 45% of supernumary embryos reached blastocysts, with an inner cell mass in 82% of them. Another recent optimistic study on blastocyst culture in which heparin-binding epidermal growth factor (Hb–EGF) was added to embryos cultured from day 2–7 post-insemination, has also …

Is the success of human IVF more a matter of genetics and evolution than growing blastocysts?