THE MONTHLY CYCLE (MENSTRUATION AND PREGNANCY)

As a girl begins to reach womanhood, her breasts begins to enlarge, and hair starts to grow in her pubic area. A year or two later she undergoes a period of rapid growth, after which she begins to menstruate. At first the intervals between menses are irregular because her ovaries are not producing mature sex cells. However, within a year  or so, the time of menstruation becomes irregular.

 A rhythmic pattern are called the 28-day female menstrual cycle, changes in the lining of the uterus (mucosa) and body temperature during a  (more accurately the female sexual cycle) continues throughout the normal reproductive years, and is regulated by rhythmic changes in the female hormones.

The ebb and flow of these hormones causes the changes seen in the sexual organ (ovary, uterus, breasts). While the average cycle is twenty eight days, the interval may vary from as little as twenty days to more than forty days in perfectly normal women.

Stimulated by hormones from the pituitary gland (follicle-stimulating hormone -- FSH, and luteinizing  hormone--LH), a few mature follicles (up to ten) in each ovary respond, but only one ovary releases an ovum (usually alternating each month between ovaries). This process,called ovulation, occurs fourteen days after sexual (menstrual) cycle. Note the abrupt drop in body temperature at the time of ovulation.menses begins,in a woman who has a twenty-eight day cycle.

The cells lining the empty follicle rapidly increase in number and become changed to lutein cells (forming the corpus luteum) which secrete progesterone and estrogen. Under the influence of these hormones, the lining of the uterus, called the endometrium,undergoes considerable change. Blood vessels increase in number, the lining cells multiply rapidly, the endometrium doubles in thickness (becoming 4 to 6 millimeters thick), growing soggy with nutritious secretions known as "uterine milk",and takes on a velvety appearance -- all in readiness to received a fertilize ovum. When conception does not occur, the level of the ovarian hormones (estrogen and progesterone) drops suddenly. Without the encouragement of these hormones the endometrium begins to deteriorate. The underlying blood vessels narrow and soon break apart. The lining cells, denied the blood they need, disintegrate. The unfertilized ovum with much of the mucous membrane is shed, accompanied by the loss of considerable blood. Contractions of the uterus (sometimes causing painful "cramps") aid in expelling this blood and tissue through the vagina. This process is called menstruation.

Typically, five days after menstruation starts  (anywhere from three to seven days is normal), blood flow ceases. A little more than 2 ounces (70 ml) of blood, cells, and tissue fluids are lost.The inside of the uterus is now completely stripped of its lining, and rebuilding of this lining begins again in anticipation of a pregnancy occurring during the next month. The enormous number of white cells present in the menstrual flow protect the uterus from possible infection. 

When conception does occur, the cells of the endometrium initially act as nursemaid to the fertilized egg when its first arrives in the womb (about four days after conception). A secretion called "uterine milk"provides the rapidly growing embryo the food it needs. After three or four days, the ovum implants itself in the lining. Still later, as the demands of the growing embryo increase, the placenta develops. The placenta is an organ which act as a go- between for the mother and fetus. Oxygen and nutrients from the mother's blood pass to the blood of the fetus, while many of the waste formed by the fetus move in the opposite direction. These waste travel through the placenta into the mother's blood, to be expelled by her eliminative organs. Some of the major events in the calendar of a twenty-eight-day menstrual cycle are summarized below. The menstrual flow continues for about five days, following which the rebuilding of the uterine lining begins and continues for eleven or twelve days (proliferative phase). Then,for another eleven to twelve days, the lining cells fill with fluid to nourish the ovum (secretory phrase). Fourteen days after the start of menstruation, ovulation occurs.Twenty-six days from the start, if the ovum is not fertilized, the lining begins to disintegrate, and two days later menstruation commences again.