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The present paper reviews evidence that preimplantation embryos are naturally exposed and designed to respond to growth factors during preimplantation development.Recent studies have demonstrated that in-vivo human preimplantation embryos are exposed to a mixture of many different growth factors, expressed by the follicles, oviducts and endometrium. Receptors for many of these growth factors have also been shown to be expressed by preimplantation embryos, suggesting a functional role during preimplantation development. Studies of in-vitro fertilization in both animals and humans indicate that in-vitro culture of embryos in conventional media lacking growth factors can result in suboptimal growth and a variety of short-term and long-term developmental abnormalities. Studies of embryo coculture indirectly suggest that growth factors can improve in-vitro development. Many studies of defined growth factor supplements demonstrate that their inclusion in culture media can substantially improve preimplantation development and efficacy of in-vitro fertilization, and may reduce long-term developmental abnormalities as well.Embryos are naturally exposed to a complex mixture of growth factors that play an important role in preimplantation embryo development and that are likely to be of substantial benefit if added to in-vitro culture media.