Your cycle is guided by a small group of hormones that rise and fall to regulate ovulation and menstruation.
Estrogen
Estrogen plays an important role in the first half of the cycle. This hormone helps rebuild the uterine lining after menstruation. As an egg develops in the ovary, estrogen levels gradually rise. This increase is part of the phase leading up to ovulation.
Progesterone
Progesterone is mainly active after ovulation, in the second half of the cycle. This hormone helps maintain the uterine lining in preparation for a possible pregnancy. If fertilization does not occur, progesterone levels fall. This drop causes the uterine lining to shed and menstruation begins.
LH, luteinizing hormone
LH plays a central role around ovulation. A sharp, short rise in this hormone triggers the release of the mature egg from the ovary. Without this LH surge, ovulation does not take place.
FSH, follicle-stimulating hormone
FSH is especially active at the beginning of the cycle. This hormone stimulates the maturation of a follicle in the ovary that contains an egg. This marks the start of a new cycle.
How these hormones work together
At the start of the cycle FSH stimulates the maturation of an egg. As this happens estrogen rises and the uterine lining rebuilds. Around the middle of the cycle a surge in LH triggers ovulation. After this progesterone supports the second half of the cycle. When progesterone drops, menstruation begins and the cycle starts again.