Two hormones, two halves of the cycle
The 28-day cycle is often summarized by two hormones taking the lead in turn. Estradiol dominates the first half, climbing as ovarian follicles mature and peaking around the time of ovulation, frequently charted near day 13. Progesterone dominates the second half, the luteal phase, rising after ovulation and typically peaking around day 21.
Progesterone comes from the corpus luteum, the structure left behind after the follicle releases its egg. Its job is to prepare and maintain the uterine lining, which is why its rise follows ovulation rather than preceding it.
What the falling tail of the curve signals
If pregnancy does not occur, the corpus luteum breaks down, so both progesterone and estradiol drop late in the luteal phase. That withdrawal triggers shedding of the lining and the start of the next cycle. The overlay therefore reads as a handoff between two hormones, then a shared decline.
The day-21 and day-13 markers describe an idealized 28-day cycle. Real cycles vary in length and timing. This is general educational information, not medical advice; consult a qualified professional for personal cycle or fertility questions.