Describe the uterine events associated with menstruation.

The menstrual cycle of the human female is approximately 28 days long and is numbered from the first day of the menses. Menstruation is the process in which the lining of the uterus is shed once during each cycle, a process that in most women takes 4 to 5 days. under influence of increasing titers of plasma estrogen, the endometrium of uterus increases in thickness, reaching a maximal width of 3 to 5 mm just prior to ovulation. Stromal connective tissue cells proliferate and extracellular collagen deposits are increased. In response to luteal phase progesterone levels, uterine glands increase in complexity from simple tubular elements during follicular phase of cycle to thick, coiled structures with a glandular lumen containing abundant secretory material. Spiral arteries within the endometrium become thickened and engorged with blood, particularly in response to luteal levels of progesterone. If implantation of a zygote fail to occur, then by about 11 days following ovulation, lymphocytes begin to invade the endometrium, and by day 14, sloughing of the endometrium occurs due to loss of ovarian steroid hormone support. In the absence of a hormonal directive, the spinal arteries become constricted and the blood lost from these arteries along with stromal debris composes the bulk of menstrual flow.
The nature of cervical mucus is also characteristically altered during the menstrual cycle. During the follicular phase, the mucus is rather thin and watery. In response to periovulatory levels of progesterone, the mucus becomes thickened and is composed of innumerable tiny channels that apparently provides access for the sperm through the cervical.