Post-Peak Phase

The post-peak phase is the part of the cycle that begins the day after Peak Day and ends the day before the next period. It corresponds to the luteal phase, the second half of the cycle, when progesterone is the dominant hormone. Peak Day is the last day of the most fertile mucus, and ovulation occurs close to it. In one study of charted cycles, ovulation fell within two days of Peak Day 95.4% of the time.¹ Once the peak symptom has passed, the corpus luteum becomes the main source of progesterone. Cervical mucus becomes scant or dry. Within a few days, the chart shifts from fertile to nonfertile.

The post-peak phase is the more consistent half of the cycle. The length of the pre-peak phase can vary considerably from one cycle to the next. The length of the post-peak phase remains relatively stable. Measured from ovulation to the next period, a normal luteal phase lasts 11 to 17 days, and most last 12 to 14 days.² Peak Day falls close to ovulation, so the post-peak count on a chart closely tracks this range. A short post-peak phase can indicate that the corpus luteum produces insufficient progesterone to support an early pregnancy. This pattern is associated with luteal phase deficiency.³

The post-peak phase is also the window for cycle-timed measurement of progesterone. Blood samples drawn on designated days after the peak symptom reveal the actual output of the corpus luteum in that cycle, rather than a population average. A short or hormonally weak post-peak phase appears on the mucus pattern chart and can be confirmed with timed laboratory tests. It is one of the earlier findings that a clinician can correct in a fertility evaluation. and dry day.