Hormone changes in the body trigger each of the four phases of the menstrual cycle: menstruation, follicular phase, ovulation and luteal phase.
Menstruation is the first phase of the menstrual cycle and is the shedding of lining of the uterus. During menstruation, the levels of the hormones estrogen and progesterone drop, which causes the endometrium to detach from the walls of the uterus and bleed.
Proliferative (follicular) phase is the phase of the menstrual cycle that starts after menstruation and ends with ovulation. During this phase, the ovaries produce follicles which develop into a mature egg. The level of estrogen increases, and this helps to thicken the lining of the uterus so that it can support a pregnancy. The follicles grow and mature under the influence of follicle-stimulating hormone (FSH).
Ovulation phase is when an egg is released from a follicle in the ovary. Ovulation is triggered by a surge in the hormone luteinizing hormone (LH). The egg travels down the fallopian tube towards the uterus, where it can be fertilized by a sperm.
Luteal phase begins after ovulation where the follicle that released the egg becomes a corpus luteum, which produces progesterone. Progesterone helps to maintain the thick lining of the uterus to prepare for a fertilized egg. If fertilization does not occur, the corpus luteum breaks down and the level of progesterone decreases. This causes the lining of the uterus to shed, and the menstrual cycle begins again.