<P> Two or three days before LH levels begin to increase, usually by day seven of the cycle, one (or occasionally two) of the recruited follicles has emerged as dominant . Many endocrinologists believe that the estrogen secretion of the dominant follicle has increased to a level that GnRH production is suppressed, which lowers the levels of LH and FSH . This slowdown in LH and FSH production leads to the atresia (death) of most of the recruited follicles, though the dominant follicle continues to mature . Estrogen levels will continue to increase for several days (on average, six days, but this varies widely). </P> <P> These high estrogen levels initiate the formation of a new layer of endometrium in the uterus, histologically identified as the proliferative endometrium . Crypts in the cervix are also stimulated to produce fertile cervical mucus . This mucus reduces the acidity of the vagina, creating a more hospitable environment for sperm . It also has a characteristic texture that helps guide sperm through the cervix and to the fallopian tubes, where they wait for ovulation . In addition, basal body temperature may lower slightly under the influence of high estrogen levels . </P> <P> Estrogen levels are highest right before the LH surge begins . The short - term drop in steroid hormones between the beginning of the LH surge and the event of ovulation may cause mid-cycle spotting or bleeding . Under the influence of the preovulatory LH surge, the first meiotic division of the oocytes is completed . The surge also initiates luteinization of thecal and granulosa cells . Ovulation normally occurs 30 (± 2) hours after the beginning of the LH surge (when LH is first detectable in urine). </P> <P> The time from recruitment of tertiary stage follicles to ovulation, is normally about two weeks, comprising days 1 - 14 of an idealized 28 - day menstrual cycle . However, it is common for the follicular phase to last much longer, as sometimes no dominant follicle is selected . In this case, more tertiary stage follicles are usually recruited, and the process begins again . One study of women with regular menstrual cycles found that none of them ovulated after the first wave of follicle development: 68% of cycles showed ovulation after the second wave, while 32% of cycles showed ovulation after the third wave . Fertility awareness sources may refer to multiple waves of follicular development as a split peak because of the characteristic pattern of cervical mucus changes that is sometimes observed . </P>

At which stage in the menstrual cycle is each hormone most active