The Menstrual Cycle

The normal human menstrual cycle is 28 days, but no woman is always precisely regular, and cycles as short as 21 days or as long as 35 days are not abnormal. It is customary to call the first day of the menstrual period the first day of the cycle, although menstruation is the end rather than the beginning of a process. On this basis the cycle is described as starting with about five days of menstruation, followed by a follicular phase that lasts to about the 14th day, and then a secretory phase that lasts until the next menstruation. The external manifestation of menstruation depends upon cyclical change in the lining of the body of the uterus. The lining, called endometrium, consists of tubular glands that open into the uterine cavity.

After ovulation the granulosa cells lining the follicle from which the ovum has been extruded accumulate yellow lipid and are therefore called lutein cells. The altered follicle is called corpus luteum. The corpus luteum continues to secrete estrogens but now also secretes progesterone; this additional hormone induces the secretory phase in the endometrium. The endometrial glands are distened with secretion and become very tortuous, while the stromal cells are swollen. The appearance of the endometrium at the end of the menstrual cycle is indistinguishable from that of early pregnancy, and this endometrial change is a preparation for the reception of the ovum. If it is fertilized, the ovum liberated at mid-cycle reaches the uterine cavity at a time when the endometrium is in the secretory phase, and the ovum embeds itself in the endometrium and starts its growth. If the ovum is not fertilized the endometrium breaks down and menstruation occurs. Menstruation has therefore been described as the outward evidence of the abortive close of one cycle and the hopful commencenment of the

Pregnancy tests are based on a detectable increase in human chronic gonadotropin (HCG) in the blood serum and urine during early pregnancy. HCG is the principal hormone produced by the chronic layers of the placenta, the temporary organ that provides nourishments for the developing fetus. Levels of HCG increases significantly following implantation of the fertilized egg in the uterine wall, which occurs sometime between 6 and 12 days after fertilization. In home pregnancy tests, which are qualitative ( determining whether HCG is present ), a small amount of urine is applied to a chemical strip. The result is usually indicated by some visible change in the strip. A positive home pregnancy test should be confirmed with a laboratory test and pelvic examination by a doctor. Pregnancy tests performed in a laboratory on a sample of blood or urine are quantitative and therefore are more accurate than a home pregnancy test. Laboratory tests using a sample of blood also have a high degree of sensitivity and can be used to detect increased levels of HCG early in the implantation