The cervix is the narrow, lower portion of the uterus that connects to the vagina. It acts as a gateway between the two, opening and closing at different times to let menstrual blood out, sperm in, and eventually a baby through during childbirth. It measures roughly 3 cm long and 3 cm wide, about the size of a small walnut, though its dimensions vary depending on age and whether someone has given birth before.
Anatomy of the Cervix
The cervix has two main regions. The ectocervix is the outer portion that protrudes into the top of the vagina. This is the part a doctor or midwife can see during a pelvic exam. At its center is a small opening called the external os, which leads into the vagina.
Running through the middle of the cervix is the endocervical canal, a narrow tunnel that connects the external os to the internal os. The internal os is the upper opening where the cervix meets the main body of the uterus. This opening becomes especially important during pregnancy, when providers monitor it to make sure it stays closed until delivery.
The two regions of the cervix are lined with different types of cells. The ectocervix is covered by flat, layered cells similar to the skin inside your cheek, while the endocervical canal is lined with a single layer of column-shaped cells that produce mucus. The spot where these two cell types meet is called the transformation zone. This area is significant because it’s where nearly all cervical cancers originate, which is why screening tests specifically sample cells from this zone.
What the Cervix Does
The cervix serves as a biological gatekeeper. Its most constant job is producing mucus that changes in consistency throughout the menstrual cycle, controlled by shifting hormone levels. These changes either help or hinder sperm from reaching the uterus.
During menstruation, the cervix opens slightly to allow blood and tissue to pass from the uterus into the vagina. For most of the cycle, cervical mucus is thick and sticky, forming a physical barrier that blocks bacteria and sperm alike. Around ovulation (roughly days 10 to 14 of a typical cycle), rising estrogen levels cause the mucus to become thin, slippery, and stretchy, resembling raw egg whites. This texture helps sperm travel through the cervical canal and into the uterus. After ovulation, the mucus dries up again, returning to its barrier state.
During pregnancy, the cervix produces a thick plug of mucus that seals the cervical canal completely, protecting the developing baby from infection. This plug stays in place until labor approaches, when it’s expelled as the cervix begins to prepare for delivery.
How the Cervix Changes During Labor
The cervix undergoes a dramatic transformation during childbirth. Two things need to happen before a baby can be delivered: effacement and dilation.
Effacement is the process of thinning and softening. The cervix starts out firm and roughly 3 cm long. As the baby’s head drops lower into the pelvis and presses against the cervix, combined with the force of uterine contractions, the cervix gradually shortens and thins until it’s essentially paper-thin. Providers measure this as a percentage, from 0% (no thinning) to 100% (fully thinned).
Dilation is the opening of the cervix. It starts completely closed at 0 cm and needs to reach 10 cm wide before the baby can pass through. Both effacement and dilation are driven by uterine contractions pushing the baby downward, creating increasing pressure on the cervix. In first-time mothers, effacement often begins before dilation does. In subsequent pregnancies, the two processes tend to happen at the same time.
Cervical Mucus and Fertility Tracking
Because cervical mucus follows a predictable pattern, many people use it to track their fertility window. In the days right after a period, discharge is minimal and feels dry or tacky, usually white or slightly yellow. As the cycle progresses into days 4 to 6, it becomes sticky and slightly damp. By days 7 to 9, the mucus takes on a creamy, yogurt-like consistency that feels wet and looks cloudy.
The clearest fertility signal arrives around days 10 to 14, when mucus becomes clear, stretchy, and very wet. This “egg white” texture indicates ovulation is near or happening, and it’s the phase when conception is most likely. After ovulation, progesterone takes over and mucus dries up again, staying that way until menstruation begins.
Common Cervical Conditions
Several benign conditions can affect the cervix. Nabothian cysts are one of the most common, harmless irregularities that gynecologists see. These small bumps form when skin cells clog mucus-producing glands in the cervix. Most people never know they have one unless a provider spots it during a routine exam. Rarely, a large cyst can cause a feeling of fullness in the vagina or discomfort during sex. Treatment is almost never needed, though a provider can remove one using heat or cold therapy if it’s causing problems or interfering with screening.
Cervical insufficiency is a pregnancy-specific condition where the cervix begins to open too early, sometimes without any contractions or pain. It’s typically identified by a history of painless dilation and second-trimester loss, or when an ultrasound reveals the cervix shortening before week 24. Treatment options include a cerclage (stitches placed around the cervix to hold it shut until the final month of pregnancy) or vaginal progesterone, a hormone supplement that can help keep the cervix from opening prematurely. Women with a history of early preterm birth are usually monitored with ultrasounds every two weeks between weeks 16 and 24.
Cervical Cancer Screening
The transformation zone, where the two cell types of the cervix meet, is vulnerable to changes caused by certain strains of human papillomavirus (HPV). Persistent infection can cause normal cells to gradually become abnormal and, over many years, potentially cancerous. Screening catches these changes early, long before cancer develops.
Current guidelines from the U.S. Preventive Services Task Force recommend no screening before age 21, regardless of sexual activity. From ages 21 to 29, a Pap test every three years is recommended. From ages 30 to 65, there are three options: a Pap test every three years, an HPV test every five years, or both tests together every five years. After age 65, screening can stop if previous results have been consistently normal. Screening is also unnecessary after a hysterectomy that included removal of the cervix, provided there’s no history of precancerous cervical changes.
The HPV vaccine has added a powerful layer of prevention. Data from Sweden and Denmark show that women vaccinated in their teens have a measurably lower risk of developing cervical cancer as adults. The vaccine works best when given before any exposure to the virus, which is why it’s recommended in early adolescence, but it can provide benefit through age 26 and in some cases up to 45.