The menstrual period occurs 14 days after ovulation, when the uterus sheds the lining it had been preparing to nourish an expected pregnancy which never occurred. High levels of estrogen and progesterone hormones from the ovary had been directing the uterus, but since there was no pregnancy, the ovary stopped producing them (“progesterone” means “to promote gestation,” i.e. pregnancy). The body gets set to try again for the next month.
First, the Hypothalamus in the brain produces Gonadotropin Releasing Hormone (GnRH), which had been on hold while estrogen and progesterone levels had been high. GnRH tells the Pituitary Gland (tucked within the brain, right nearby) to release Follicle Stimulating Hormone (FSH). The FSH, as its name implies, tells the ovary to make estrogen again and get another follicle ready to produce an egg (oocyte) (a baby girl is born with over a million immature ones in her ovaries). It takes an average of 2 weeks (1 to 3) for the oocyte to mature. This is the “Follicular Phase”.
For some unknown reason, right at midcycle, the rising amounts of estrogen paradoxically tell the Pituitary to release a large amount of Luteinizing Hormone (LH), without any increase in GnRH (this is one of the unsolved mysteries of the human body). The LH surge is responsible for the release of the oocyte from its follicle. The empty ovarian follicle seals off and turns into a mass of cells called the Corpus Luteum, which produces Progesterone during the “Luteal Phase”. But when no baby is conceived, production stops after 14 days, and the Uterus sheds its Endometrial lining (menstruation).
Certain brain diseases can interfere with this process by interrupting GnRH, FSH, and LH production, causing Amenorrhea (no menstrual periods). A main one are Pituitary Gland tumors, usually benign. Most of these are microscopic, but a large one could be dangerous; we always obtain an MRI of the Pituitary Gland if the blood Prolactin level is elevated, and sometimes if we don’t have another diagnosis for the amenorrhea. We’d certainly look for a large tumor if the woman has new headaches, or abnormal visual fields on an eye exam.
Various factors can inhibit the Hypothalamus: eating disorders, strenuous exercise, nutritional deficiencies (especially avoidance of fat), Celiac Disease, Type-1 Diabetes, rare tumors, and major physical or emotional stress. Other diseases of the Pituitary Gland besides the common small benign tumors are rare cancerous ones, brain radiation treatment (not CT scans), and a number of unusual conditions such as Sheehan’s Syndrome (loss of circulation from major hemorrhage after delivery); Hemochromatosis (iron overload); Sarcoidosis; Stroke; (and more).
See also our topic Amenorrhea.