The primary ethical problem relates to the use of embryonic stem cells. Under the directive, these are cells created prior to 1 June 2002 only for infertility purposes, which are described as supernumerary embryos. These embryos have not been created specifically for research. The Oviedo convention condemns this but Ireland has not signed it. This is difficult to follow. It has long been agreed it is unethical to produce embryos for research but, when a couple decides to undergo treatment for infertility, it may be necessary for medical reasons to fertilise the woman's egg with the man's sperm outside the man in a little glass dish. This is known as in vitro fertilisation. Each fertilised egg develops into a ball of cells called a blastocyst. Approximately three were placed in the woman's womb in the past, as this was considered to be the best way to produce one or two children. However, advances in technology mean that only one or two blastocysts must be inserted. The remaining fertilised eggs are frozen in many countries to avoid the hazards of multiple children which may develop for both the mother and children.