The menstruation cycle or hormonal cycle facilitates maturation and rupture of the ovarian follicle resulting in changes that take place, which prepare the uterus for pregnancy.
The first day of menstruation (referred to as Day 1) occurs when levels of estrogen and progesterone are low. In response to these low levels, the hypothalamus secretes gonadotropin releasing hormone (GnRH) which triggers the anterior pituitary gland to release two hormones: follicle stimulating hormone (FSH), and luteinizing hormone (LH).
FSH stimulates the development of many follicles within the ovary. Usually, one dominant follicle takes over. As it continues to grow, it produces increasing amounts of estrogen, which stimulates the release of LH, and inhibits FSH, which suppresses further follicular development.
When LH levels are highest (LH surge), the ovarian follicle “ruptures” and releases one ovum, which is “swept” into the fallopian tube by hair like projections at the end of the tubes called fimbriae. This process is called ovulation. Increasing estrogen levels causes the cervical mucus (vaginal secretions) to become clear and profuse and the so to dilate. These two actions facilitate the transport of semen (containing sperm) from the vagina, through the uterus, and into the fallopian tube.
Following ovulation, the ruptured follicle is transformed into the corpus luteum, a glandular mass that continues to produce estrogen and high levels of progesterone. The progesterone causes the endometrium to thicken, preparing it for implantation of a fertilized egg. If fertilization takes place during ovulation, hormonal levels remain high, essential for the maintenance of the pregnancy.
If fertilization does not occur, the corpus luteum shrinks and levels of both estrogen and progesterone decrease. The withdrawal of estrogen and progesterone cause the blood vessels of the endometrial (uterine) lining to “break” resulting in vaginal bleeding (menstruation). The average menstrual cycle is 28-35 days, and menstrual flow usually continues for three to seven days, although there are normal variations among women.
Following menstruation, estrogen and progesterone levels are low, triggering the hypothalamus to once again release GnRH, starting the entire cycle again. If fertilization does take place, menstruation will not reoccur for the duration of the pregnancy.