The myometrium, which consists of smooth muscle cells, responds to steroid hormones. These muscle cells contract to facilitate movement of substances from the cervix to the fundus during ovulation. They also aid in the rapid transport of sperm from the cervix to the oviducts. During menstruation, contractions occur from the fundus to the cervix, assisting in the expulsion of the sloughed stratum functionalis. Estrogen and progesterone influence the size and number of smooth muscle cells. In healthy cycling women, the myometrium remains robust, while it gradually thins in postmenopausal women. Pregnancy brings about significant changes, as the smooth muscle cells increase in length from 50 to 500 µm. Additionally, the pregnant myometrium contains a higher number of smooth muscle cells and more extracellular matrix.

Various disorders of menstruation are relatively common. These include menorrhagia (excessive menstrual flow resulting in the loss of more than 80 mL of blood), metrorrhagia (irregular and sometimes prolonged menstrual flow between normal periods), and dysmenorrhea (painful periods). Oligomenorrhea refers to the occurrence of a few irregular periods, while amenorrhea indicates the absence of periods. These conditions are often due to dysfunction of the hypothalamic-pituitary-ovarian axis rather than local pelvic pathophysiology.

Due to the natural shedding of endometrial tissue in fragments containing viable cells, this tissue can sometimes access other parts of the female tract, such as the oviducts, ovary, and the lower abdomen and associated structures. This occurrence leads to the development of endometriosis, which is hormonally responsive endometrial tissue outside the uterus. The spread of endometriosis may be attributed to reflux of menstrual tissue into the oviducts or the movement of tissue through lymphatics, or both. Endometriosis often causes cyclic bleeding and is associated with infertility, pain on defecation, pain on urination, pain with sexual intercourse, or generalized pelvic pain.

The cervix, which is the lower part of the uterus projecting into the vagina, possesses a mucosa lining the endocervical canal. This canal has a highly elastic lamina propria and a muscularis that is continuous with the myometrium. The part of the cervix extending into the vaginal vault is known as the ectocervix, while the part surrounding the endocervical canal is called the endocervix. The openings of the endocervical canal at the uterus and vagina are referred to as the internal cervical os and external cervical os, respectively. The cervix acts as a gateway to the upper female tract. During midcycle, the endocervical canal facilitates sperm viability and entry. In the luteal phase, the endocervical canal impedes the passage of sperm and microbes, thereby preventing the superimplantation of a second embryo or ascending infection into the placenta, fetal membranes, and fetus. The cervix also provides physical support for the growing fetus. At term, cervical softening and dilation allow for the passage of the newborn and placenta from the uterus into the vagina.

The hormonal regulation of cervical mucus occurs throughout the menstrual cycle.
The smooth muscle cells of the myometrium are also responsive to changes in steroid hormones. Peristaltic contractions of the myometrium favor movement of the luminal contents from the cervix to the fundus at ovulation, and these contractions probably play a role in rapid bulk transport of ejaculated sperm from the cervix to the oviducts. During menstruation, contractions propagate from the fundus to the cervix, thereby promoting expulsion of sloughed stratum functionalis. The size and number of smooth muscle cells are determined by estrogen and progesterone. Healthy cycling women maintain a robust myometrium, whereas the myometrium progressively thins in postmenopausal women. The most drastic changes are seen during pregnancy, when the smooth muscle cells increase from 50 to 500 µm in length. The pregnant myometrium also has a greater number of smooth muscle cells and more extracellular matrix.
Disordersofmenstruationarerelativelycommonandincludemenorrhagia (heavymenstrualflowleadingtolossofmorethan80mLofblood),metrorrhagia irregularandsometimesprolongedmenstrualflowbetweennormalperiods),anddysmenorrhea (painfulperiods).Theexistenceofafewirregularperiods,calledoligomenorrhea, andtheabsenceofperiods,calledamenorrhea, areoftenduetodysfunctionofthehypothalamic-pituitary-ovarianaxisasopposedtolocalpelvicpathophysiology.
Because endometrial tissue is naturally sloughed in fragments that contain viable cells, endometrial tissue occasionally gains access to other parts of the female tract (e.g., oviducts, ovary), as well as the lower part of the abdomen and associated structures (e.g., rectouterine pouch of Douglas, as shown in
Fig. 44.13 ). These implants give rise to endometriosis—a foci of hormonally responsive endometrial tissue outside the uterus. The spread of endometriosis may be due to reflux of menstrual tissue into the oviducts or movement of tissue through lymphatics, or both. Endometriosis frequently exhibits cyclic bleeding and is associated with infertility, pain on defecation, pain on urination, pain with sexual intercourse, or generalized pelvic pain.
The Cervix
The cervix is the inferior extension of the uterus that projects into the vagina (see
Figs. 44.13 44.25 ). It has a mucosa that lines the endocervical canal, which has a highly elastic lamina propria and a muscularis that is continuous with the myometrium. The part of the cervix that extends into the vaginal vault is called the ectocervix, whereas the part surrounding the endocervical canal is called the endocervix. The openings of the endocervical canal at the uterus and vagina are called the internal cervical os and the external cervical os, respectively. The cervix acts as a gateway to the upper female tract—at midcycle the endocervical canal facilitates sperm viability and entry. During the luteal phase the endocervical canal impedes passage of sperm and microbes, thereby inhibiting superimplantation of a second embryo or ascending infection into the placenta, fetal membranes, and fetus. The cervix physically supports the weight of the growing fetus. At term, cervical softening and dilation allow passage of the newborn and placenta from the uterus into the vagina.
Hormonal Regulation of Cervical Mucus During the Menstrual Cycle