The cervix connects the uterus to the vagina. This muscular, tunnel-like organ regulates the passage of menstrual blood and blocks foreign substances, such as bacteria, from entering the upper reproductive tract. Typically a few centimeters in length, the cervix undergoes dramatic transformations once pregnancy begins. These changes in position, texture, and structure are necessary to maintain the pregnancy and prepare the body for childbirth.
The Cervix’s Position and Function During Pregnancy
Immediately upon conception, hormonal signals begin to transform the cervix from a firm, closed structure. The cervix often shifts position, moving higher up within the vaginal canal and frequently tilting backward (posterior). This upward and backward movement helps secure the uterus and keeps the cervix out of the way of the growing fetal head and internal pressure.
Early in the first trimester, the cervix’s texture softens significantly, a phenomenon known as Goodell’s sign. Due to increased blood flow and hormonal influence, the tissue changes from a consistency similar to the tip of the nose to something much softer, like an earlobe. This physical change is accompanied by the formation of the mucus plug (operculum), a dense, sticky collection of cervical secretions.
The mucus plug seals the cervical canal, acting as a physical and chemical barrier. This protective seal prevents bacteria and other pathogens from ascending into the sterile environment of the uterus and amniotic sac. For the duration of the pregnancy, the cervix remains long, closed, and sealed to maintain the integrity of the gestation.
Monitoring Cervical Health and Length
Throughout gestation, healthcare providers monitor the cervix to assess its health. While a manual digital exam provides information about softness and position, objective measurement is performed using transvaginal ultrasound (TVUS). This technique provides a clear, precise image of the cervical canal, allowing clinicians to measure its functional length in millimeters.
Cervical length measurement is a standard tool used to screen for the risk of preterm birth, especially in the second trimester. A typical cervical length is around 30 to 50 millimeters; a measurement falling below 25 millimeters before 24 weeks of gestation is considered a short cervix, indicating a higher risk. This objective data helps guide decisions about prophylactic treatments, such as progesterone or a cerclage.
Another finding assessed during the TVUS is “funneling,” which is the widening of the internal cervical opening closest to the uterus. This ballooning of the fetal membranes into the cervical canal suggests that the internal seal is beginning to fail. Funneling, particularly when combined with a shortened length, can be a sign of cervical insufficiency. This condition involves the cervix painlessly dilating and thinning without contractions, potentially leading to late miscarriage or preterm delivery.
The Cervix’s Role in Preparing for Labor
The final transformation of the cervix begins in the weeks leading up to and during labor, a process called cervical ripening. The cervix changes from a firm, long barrier to a soft, short, and open passageway. This change involves two distinct, measurable processes: effacement and dilation.
Effacement refers to the shortening and thinning of the cervical canal, measured in percentages from 0% (long and thick) to 100% (paper-thin). Dilation is the opening of the internal and external cervical os, measured in centimeters from closed to 10 centimeters, which is considered fully dilated. These coordinated changes allow the baby to pass from the uterus into the birth canal.
The progress of these changes is often clinically summarized using the Bishop Score, an assessment system that predicts the likelihood of successful labor induction. The Bishop Score evaluates five factors:
- Cervical dilation
- Effacement
- Consistency
- Position
- Fetal station
Fetal station refers to how far the baby’s head has descended relative to the mother’s pelvis. A lower station indicates the baby is more engaged and closer to being born.