<P> Several days after ovulation, the increasing amount of estrogen produced by the corpus luteum may cause one or two days of fertile cervical mucus, lower basal body temperatures, or both . This is known as a "secondary oestrogen surge". </P> <P> The hormones produced by the corpus luteum also suppress production of the FSH and LH that the corpus luteum needs to maintain itself . With continued low levels of FSH and LH, the corpus luteum will atrophy . The death of the corpus luteum results in falling levels of progesterone and oestrogen . These falling levels of ovarian hormones cause increased levels of FSH, which begins recruiting follicles for the next cycle . Continued drops in levels of oestrogen and progesterone trigger the end of the luteal phase: menstruation and the beginning of the next cycle . </P> <P> The human luteal phase lasts between ten and sixteen days, the average being fourteen days . Luteal phases of less than twelve days may make it more difficult to achieve pregnancy . While luteal phase length varies significantly from woman to woman, for the same woman the length will be fairly consistent from cycle to cycle . </P> <P> The loss of the corpus luteum can be prevented by implantation of an embryo: after implantation, human embryos produce human chorionic gonadotropin (hCG), which is structurally similar to LH and can preserve the corpus luteum . Because the hormone is unique to the embryo, most pregnancy tests look for the presence of hCG . If implantation occurs, the corpus luteum will continue to produce progesterone (and maintain high basal body temperatures) for eight to twelve weeks, after which the placenta takes over this function . </P>

What is the maximum length of luteal phase