 Succesful implantation requires a co-ordinated series of events allowing a timely dialogue between a receptive endometrium and the intrusive blastocyst Tabibzadeh, 1998. The period of receptivity is thought to be 3 days in human Rogers et al, 1989; Yoshinaga, 1988; Psychoyos, 1993; Harper, 1992. It is suggested that blastocyst apposition begins about day LH6 and is completed by day LH10 Lessey, 2000. 