By acting on the hypothalamus, progesterone also causes the BBT to rise around the time of ovulation. Estrogen and progesterone also cause changes in the position and feel of the cervix itself. As ovulation approaches, the opening to the cervix becomes softer and wider, and the cervix pulls up higher in the vaginal vault. After ovulation, the cervix returns to a lower position, and its opening closes and feels more firm. Because fluctuating levels of hormones make fertility signs imprecise markers of the beginning and end of the fertile time, women must abstain or use a barrier method for several days longer than the actual fertile phase.
Women can use calendar calculations to identify the fertile time.
These calculations were developed in the 1930s to estimate the days the woman is fertile in the current cycle based upon her past cycle lengths. The calculations take into account the lifespan of the egg and sperm and allow for variation in the timing of ovulation from one cycle to the next. If a woman uses a calendar calculation, the interval of time identified as potentially fertile is also longer than the actual time she is fertile. The more a woman's cycle length varies, the longer is the interval identified as potentially fertile. Recent research shows that the actual fertile time lasts for only about six days each cycle.