Figure 1.

Estradiol and progesterone levels across the menstrual cycle, frequently sampled in 47 healthy women, 18–42 years old, with self-reported history of regular menstrual cycles, and no hormonal use. Cycle phase staging was accomplished by forward counting from onset of menstrual cycle (days 1–7), backward counting from day of LH surge (days 8–11, and days 12–14), forward counting from LH surge (days 15–18, and days 19–23) and backward counting from onset of next menses (days 24–28).