Menstruation occurs when blood and the lining of the uterus is shed at the beginning of the menstrual cycle. A woman's menstrual period lasts from three to seven days and the menstrual cycle averages 28 to 32 days, the American Pregnancy Association states. Hormone fluctuations cause the changes that occur during the menstrual cycle, and the menstrual cycle is divided into two parts: the ovarian cycle and the uterine cycle.
Follicular Phase
The follicular phase is the first phase of the ovarian cycle and starts the first day of a woman's last menstrual period. The follicular phase (days 1 to 13) continues until ovulation. Follicular stimulating hormone (FSH) is produced and begins the process of maturing the eggs for ovulation. It is referred to as the follicular phase because during this time, the egg grows and matures inside of a small sac known as the follicle. The follicles inside the sac enlarge and produce estrogen, which triggers a surge of lutening hormone (LH). LH causes the mature egg to be released from the follicle. This is known as ovulation.
Luteal Phase
After ovulation, the second phase of the ovarian cycle called the luteal phase begins and lasts for 12 to 16 days. The luteal phase ends when the first day of menstruation occurs, the American Pregnancy Association states. After the egg is released, the corpus luteum, which is an endocrine structure necessary for achieving and maintaining pregnancy, remains in the follicle and produces progesterone. If the corpus luteum does not receive the hormone hCG from a developing embryo, the corpus luteum only survives for 12 to 16 days. When the corpus luteum dies, the levels of estrogen and progesterone hormones drop, which causes the uterus to start shedding its lining and causes a woman to have her menstrual period.
Uterine Cycle
According to the website Net Wellness, the uterine cycle occurs at the same time the ovulation cycle occurs. The uterine cycle is also divided into two phases. During the proliferative phase, the uterine lining grows thicker and uterine glands multiply and grow. This phase of the uterine cycle occurs between the menstrual period and the next time a woman ovulates. The secretory phase of the uterine cycle occurs is the time after ovulation when the corpus luteum dies and the uterus sheds its lining.
