When a woman is treated for infertility it is necessary to nurture human embryos for a few days (until the first cell divisions of the fertilized egg have occurred) in a specialized laboratory. More eggs are fertilized than are usually needed because not all fertilizations are successful. Surplus embryos may be frozen for use in later attempts to implant an embryo in the womb. Research has been done on very early embryos but the Practice is controversial and some countries have either forbidden it or imposed tight restrictions. In the UK such research is controlled by the human fertilization and embryology authority.