The cervix is the cylindrical, muscular lower portion of the uterus that connects the main uterine body to the vagina. This structure facilitates the passage of sperm into the uterus and maintains its closure during pregnancy. Clinically, “small” primarily refers to a short cervical length, a condition medical professionals monitor closely. A short cervix is associated with certain pregnancy risks and affects the ability to carry a baby to term.
Defining Cervical Size
During pregnancy, a small cervix is medically defined as a short cervical length, a measurable factor associated with an increased risk for premature delivery. For a singleton pregnancy, the cervix is typically 30 to 50 millimeters long during the second trimester. A cervix is considered short when it measures less than 25 millimeters before 24 weeks of gestation. This measurement is most accurately obtained using a transvaginal ultrasound, which provides a clear view of the entire cervical canal. The transvaginal approach is the most reliable method, as it avoids distortion from the abdominal wall or the position of the fetus.
Causes and Contributing Factors
A short cervix results from a combination of congenital characteristics and acquired conditions that affect cervical tissue integrity. Some individuals are born with a naturally shorter cervix, a structural variation that may predispose them to shortening during pregnancy.
Acquired factors often involve damage or trauma to the cervical tissue from past procedures or events. Surgical treatments for abnormal Pap smears, such as a Loop Electrosurgical Excision Procedure (LEEP) or a cone biopsy, remove a portion of tissue and shorten the overall length. Physical trauma from a previous difficult childbirth or uterine procedures, like a dilation and curettage (D&C), may also weaken the structure. Chronic inflammation or infection can sometimes contribute to structural changes leading to a shorter cervix.
Implications for Fertility and Pregnancy
A short cervix generally has minimal impact on the ability to conceive, as its length does not typically block the passage of sperm. Fertility concerns arise primarily with severe cervical stenosis, a narrowing of the cervical opening that physically impedes sperm from entering the uterus.
Once pregnancy is achieved, however, a short cervical length becomes a significant factor for the remaining gestation. The primary implication is an increased risk of preterm labor and delivery (birth before 37 weeks). A short cervix may signal cervical insufficiency, where the cervix shortens, thins, and opens prematurely under the pressure of the growing fetus and amniotic fluid. The shorter the mid-trimester measurement, the higher the likelihood the cervix will not remain closed until full-term. This premature opening can lead to early delivery, which is the leading cause of newborn death and long-term health complications.
Management and Monitoring
Once a short cervix is identified, typically between 18 and 24 weeks during a routine ultrasound, specialized monitoring and management options are considered. For individuals considered at high risk, such as those with a history of preterm birth, serial transvaginal ultrasounds may be performed to track the cervical length for any progressive shortening. This close surveillance allows for timely intervention before significant shortening occurs.
Medical Interventions
One common medical intervention involves the use of progesterone, often administered as a vaginal suppository or gel daily until the late stages of pregnancy. Progesterone supplementation reduces the rate of spontaneous preterm birth in women with a short cervix, acting to relax the uterine muscle and strengthen the cervical barrier.
Surgical Interventions
A surgical option known as a cervical cerclage may be recommended if the cervix is particularly short (e.g., less than 10 to 15 millimeters) or if the individual has a history of second-trimester loss. This procedure involves placing a strong suture around the cervix to keep it firmly closed, providing mechanical support until the stitch is removed near the end of the pregnancy.