The menstrual cycle begins with the first day of bleeding and ends with the beginning of next bleeding.
Menstrual cycle has two phases, follicular and secretory. During the follicular phase, follicle that houses
egg grows, mainly due to estrogen influence. The most developed follicle bursts, releasing the egg cell,
during the process called ovulation. The remaining follicle transforms into corpus luteum and excretes
progesterone that regulates the second phase of the menstrual cycle, secretory phase. The secretory
phase of the menstrual cycle is characterized by thickening of uterine lining in order to house possibly
fertilized egg cell, namely embryo.