The cervix or cervix uteri (Latin, 'neck of the uterus') is the lower part of the uterus in the human female reproductive system. The cervix is usually 2 to 3 cm long (~1 inch) and roughly cylindrical in shape, which changes during pregnancy. The narrow, central cervical canal runs along its entire length, connecting the uterine cavity and the lumen of the vagina. The opening into the uterus is called the internal os, and the opening into the vagina is called the external os. The lower part of the cervix, known as the vaginal portion of the cervix (or ectocervix), bulges into the top of the vagina. The cervix has been documented anatomically since at least the time of Hippocrates, over 2,000 years ago. The cervical canal is a passage through which sperm must travel to fertilize an egg cell after sexual intercourse. Several methods of contraception, including cervical caps and cervical diaphragms, aim to block or prevent the passage of sperm through the cervical canal. Cervical mucus is used in several methods of fertility awareness, such as the Creighton model and Billings method, due to its changes in consistency throughout the menstrual period. During vaginal childbirth, the cervix must flatten and dilate to allow the fetus to progress along the birth canal. Midwives and doctors use the extent of the dilation of the cervix to assist decision-making during childbirth. The cervical canal is lined with a single layer of column-shaped cells, while the ectocervix is covered with multiple layers of cells topped with flat cells. The two types of epithelia meet at the squamocolumnar junction. Infection with the human papillomavirus (HPV) can cause changes in the epithelium, which can lead to cancer of the cervix. Cervical cytology tests can often detect cervical cancer and its precursors, and enable early successful treatment. Ways to avoid HPV include avoiding sex, using condoms, and HPV vaccination. HPV vaccines, developed in the early 21st century, reduce the risk of cervical cancer by preventing infections from the main cancer-causing strains of HPV.
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