Peak Day

Peak Day is the last day of a cycle with clear, stretchy, or slippery cervical mucus. This mucus often stretches like raw egg white, and it indicates the fertile days. Peak Day is the main marker of ovulation in mucus-based fertility awareness methods, including the Creighton Model FertilityCare System.¹ The phrase "peak fertility" often describes these days, because this type of mucus appears when the chance of pregnancy is highest.²

Peak Day closely tracks the timing of ovulation. In one study, women trained in the method charted their cycles, and hormone measurements estimated the day of ovulation. Ovulation fell within two days before or after Peak Day in 95.4% of those cycles.³⁴ A woman can identify Peak Day only on the following day, once the fertile-type mucus has stopped.³ Her chart then counts each subsequent day from Peak Day.⁵

In NaProTECHNOLOGY, Peak Day anchors the timing of blood tests. Clinicians measure progesterone and estradiol in the days after Peak Day. These measurements assess the function of the second half of the cycle, the luteal phase.⁶ The clinician sets the exact test days. Without a chart, the Peak Day of a woman remains unknown. Hormone tests then have no firm link to the time of ovulation.² Accurate identification of Peak Day is the foundation for NaProTECHNOLOGY hormone support and for the value of the entire chart.⁵

Is "peak fertility" the same as Peak Day? The two phrases often describe the same time. Peak Day has a narrower meaning. It is the single last day of peak-type mucus. A woman or a chart reviewer can identify it only after the mucus changes, by retrospective review of the chart.²