The great majority of currently existing embryonic stem cell lines originate from embryos discarded following IVF. A woman undergoing IVF receives hormone therapy to stimulate the development and maturation of multiple oocytes (eggs). After retrieval, the oocytes are fertilized with semen in culture media. In most countries where IVF is practised, on average five to ten embryos are produced, one or two of which are transferred to the woman’s uterus in an attempt to initiate a pregnancy. The remaining embryos are cryopreserved in nitrogen freezers. If an attempt to achieve a pregnancy fails, one or two embryos can be thawed for another attempt. The cryopreservation of several embryos has the advantage that it reduces the risk of twin or triplet pregnancies. One can implant one embryo per attempt. If an attempt fails, one can simply thaw another embryo and try again. Another (related) major advantage of this way of doing IVF is that the woman does not have to undergo the hormone therapy and egg retrieval procedure, which entail some risk and discomfort, before each attempt to generate a pregnancy. Though some hormones need to be administered to prepare the endometrium for implantation, it is not necessary to administer additional hormones to stimulate the ovaries.
Typically individuals or couples embarking on an IVF treatment must indicate one of the following three options for handling of any surplus embryos, that is, embryos no longer wanted by the individual or couple, usually because their wish for a child has been fulfilled: (1) anonymous donation to other infertile couples, (2) donation to scientific research, or (3) letting the embryos perish. Note that the two last options both involve destruction of the embryo, and thus prevent it from developing into a child. The great majority of existing embryonic stem cell lines has been obtained from surplus IVF embryos donated for research—as under option (2). I will refer to such embryos as discarded embryos.