<P> Fertilization by a spermatozoon, when it occurs, usually takes place in the ampulla, the widest section of the fallopian tubes . A fertilized egg immediately begins the process of embryogenesis, or development . The developing embryo takes about three days to reach the uterus and another three days to implant into the endometrium . It has usually reached the blastocyst stage at the time of implantation . </P> <P> In some women, ovulation features a characteristic pain called mittelschmerz (German term meaning middle pain). The sudden change in hormones at the time of ovulation sometimes also causes light mid-cycle blood flow . </P> <P> The luteal phase is the final phase of the ovarian cycle and it corresponds to the secretory phase of the uterine cycle . During the luteal phase, the pituitary hormones FSH and LH cause the remaining parts of the dominant follicle to transform into the corpus luteum, which produces progesterone . The increased progesterone in the adrenals starts to induce the production of estrogen . The hormones produced by the corpus luteum also suppress production of the FSH and LH that the corpus luteum needs to maintain itself . Consequently, the level of FSH and LH fall quickly over time, and the corpus luteum subsequently atrophies . Falling levels of progesterone trigger menstruation and the beginning of the next cycle . From the time of ovulation until progesterone withdrawal has caused menstruation to begin, the process typically takes about two weeks, with 14 days considered normal . For an individual woman, the follicular phase often varies in length from cycle to cycle; by contrast, the length of her luteal phase will be fairly consistent from cycle to cycle . </P> <P> The loss of the corpus luteum is prevented by fertilization of the egg . The syncytiotrophoblast, which is the outer layer of the resulting embryo - containing structure (the blastocyst) and later also becomes the outer layer of the placenta, produces human chorionic gonadotropin (hCG), which is very similar to LH and which preserves the corpus luteum . The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy . Most pregnancy tests look for the presence of hCG . </P>

What is the third phase of the ovarian cycle called and what occurs during this time