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Function

Fertility

The cervical canal is a pathway through which sperm enter the uterus after sexual intercourse,<ref name=GUYTONHALL2005>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> and some forms of artificial insemination.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Some sperm remains in cervical crypts, infoldings of the endocervix, which act as a reservoir, releasing sperm over several hours and maximising the chances of fertilisation.<ref name=BRANNIGAN2008>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> A theory states the cervical and uterine contractions during orgasm draw semen into the uterus.<ref name=GUYTONHALL2005 /> Although the "upsuck theory" has been generally accepted for some years, it has been disputed due to lack of evidence, small sample size, and methodological errors.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Some methods of fertility awareness, such as the Creighton model and the Billings method involve estimating a woman's periods of fertility and infertility by observing physiological changes in her body. Among these changes are several involving the quality of her cervical mucus: the sensation it causes at the vulva, its elasticity (Spinnbarkeit), its transparency, and the presence of ferning.<ref name="Weschler"/>

Cervical mucus

Several hundred glands in the endocervix produce 20–60 mg of cervical mucus a day, increasing to 600 mg around the time of ovulation. It is viscous as it contains large proteins known as mucins. The viscosity and water content varies during the menstrual cycle; mucus is composed of around 93% water, reaching 98% at midcycle. These changes allow it to function either as a barrier or a transport medium to spermatozoa. It contains electrolytes such as calcium, sodium, and potassium; organic components such as glucose, amino acids, and soluble proteins; trace elements including zinc, copper, iron, manganese, and selenium; free fatty acids; enzymes such as amylase; and prostaglandins.<ref name=CERVIX2006>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> Its consistency is determined by the influence of the hormones estrogen and progesterone. At midcycle around the time of ovulation—a period of high estrogen levels— the mucus is thin and serous to allow sperm to enter the uterus, and is more alkaline and hence more hospitable to sperm.<ref name=BRANNIGAN2008 /> It is also higher in electrolytes, which results in the "ferning" pattern that can be observed in drying mucus under low magnification; as the mucus dries, the salts crystallize, resembling the leaves of a fern.<ref name = Weschler>|page=58-59</ref> The mucus has stretchy character described as Spinnbarkeit most prominent around the time of ovulation.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

At other times in the cycle, the mucus is thick and more acidic due to the effects of progesterone.<ref name=BRANNIGAN2008 /> This "infertile" mucus acts as a barrier to sperm from entering the uterus.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> Women taking an oral contraceptive pill also have thick mucus from the effects of progesterone.<ref name=BRANNIGAN2008 /> Thick mucus also prevents pathogens from interfering with a nascent pregnancy.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

A cervical mucus plug, called the operculum, forms inside the cervical canal during pregnancy. This provides a protective seal for the uterus against the entry of pathogens and against leakage of uterine fluids. The mucus plug is also known to have antibacterial properties. This plug is released as the cervix dilates, either during the first stage of childbirth or shortly before.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> It is visible as a blood-tinged mucous discharge.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref>

Childbirth

The cervix plays a major role in childbirth. As the fetus descends within the uterus in preparation for birth, the presenting part, usually the head, rests on and is supported by the cervix.<ref name=WILLIAMS2005/> As labour progresses, the cervix becomes softer and shorter, begins to dilate, and rotates to face anteriorly.<ref name=GOLDENBERG2008 /> The support the cervix provides to the fetal head starts to give way when the uterus begins its contractions. During childbirth, the cervix must dilate to a diameter of more than {{safesubst:#invoke:convert|convert}} to accommodate the head of the fetus as it descends from the uterus to the vagina. In becoming wider, the cervix also becomes shorter, a phenomenon known as effacement.<ref name=WILLIAMS2005 />

Along with other factors, midwives and doctors use the extent of cervical dilation to assist decision making during childbirth.<ref name=NICE-labour><NICE (2007). Section 1.6, Normal labour: first stage</ref><ref name="NICE 2007">NICE (2007). Section 1.7, Normal labour: second stage</ref> Generally, the active first stage of labour, when the uterine contractions become strong and regular,<ref name=NICE-labour/> begins when the cervical dilation is more than {{safesubst:#invoke:convert|convert}}.<ref>{{#invoke:citation/CS1|citation |CitationClass=news }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The second phase of labor begins when the cervix has dilated to {{safesubst:#invoke:convert|convert}}, which is regarded as its fullest dilation,<ref name=WILLIAMS2005>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> and is when active pushing and contractions push the baby along the birth canal leading to the birth of the baby.<ref name="NICE 2007"/> The number of past vaginal deliveries is a strong factor in influencing how rapidly the cervix is able to dilate in labour.<ref name="WILLIAMS2005" /> The time taken for the cervix to dilate and efface is one factor used in reporting systems such as the Bishop score, used to recommend whether interventions such as a forceps delivery, induction, or Caesarean section should be used in childbirth.<ref name="WILLIAMS2005" />

Cervical incompetence is a condition in which shortening of the cervix due to dilation and thinning occurs, before term pregnancy. Short cervical length is the strongest predictor of preterm birth.<ref name=GOLDENBERG2008>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Contraception

Several methods of contraception involve the cervix. Cervical diaphragms are reusable, firm-rimmed plastic devices inserted by a woman prior to intercourse that cover the cervix. Pressure against the walls of the vagina maintain the position of the diaphragm, and it acts as a physical barrier to prevent the entry of sperm into the uterus, preventing fertilisation. Cervical caps are a similar method, although they are smaller and adhere to the cervix by suction. Diaphragms and caps are often used in conjunction with spermicides.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> In one year, 12% of women using the diaphragm will undergo an unintended pregnancy, and with optimal use this falls to 6%.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Efficacy rates are lower for the cap, with 18% of women undergoing an unintended pregnancy, and 10–13% with optimal use.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Most types of progestogen-only pills are effective as a contraceptive because they thicken cervical mucus making it difficult for sperm to pass along the endocervical canal.<ref name=FPA-POP>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In addition, they may also sometimes prevent ovulation.<ref name=FPA-POP/> In contrast, contraceptive pills that contain both oestrogen and progesterone, the combined oral contraceptive pills, work mainly by preventing ovulation.<ref name=FPA-COC/> They also thicken cervical mucus and thin the lining of the uterus enhancing their effectiveness.<ref name=FPA-COC>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>


Cervix sections
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