^IIn vitro^i fertilisation (IVF) technique performed at the IVF unit, Cromwell Hospital, London, showing technician examining a petri dish (containing the ova and sperm) through an optical microscope, (sperm is introduced through the pipette) to check for successful fertilisation. Initially, 2-3 ova are suspended in a liquid medium (the pink colour is due to an acid-base indicator - pH & temperature are critical): sperm (previously collected & stored in liquid nitrogen) is brought to ambient temp. & mixed with the ova. To minimise contamination risk, IVF is performed in a ultra-clean area, ie a dust-free cabinet.