The cervix, the lower part of the uterus, normally remains long, firm, and closed throughout pregnancy. A short cervix is identified when its length measures less than 25 millimeters (2.5 centimeters) before 24 weeks of gestation, typically via transvaginal ultrasound. This physical shortening is strongly linked to an increased risk of spontaneous preterm birth (delivery before 37 weeks). Premature shortening indicates the cervical tissue has a decreased ability to withstand the mechanical and biochemical stresses of pregnancy, potentially leading to early dilation and labor.
Structural Factors and Surgical History
Cervical shortening often results from physical alterations to the tissue caused by prior medical procedures. Excisional procedures used to treat high-grade cervical dysplasia, such as the Loop Electrosurgical Excision Procedure (LEEP) or cold knife conization, remove a cone-shaped section of cervical tissue. This surgical removal directly reduces the overall functional length and structural integrity of the cervix. The degree of subsequent shortening and risk is often proportional to the volume or depth of the tissue removed.
Surgical Impact
The cervix is primarily composed of connective tissue, and the removal of even a small amount can compromise its ability to remain closed under increasing intrauterine pressure. Furthermore, the resulting scar tissue may be less elastic and structurally sound than the original tissue.
Congenital Factors
Less commonly, a short cervix may stem from congenital anomalies, which are structural defects present from birth. These defects arise from abnormal development of the Müllerian ducts during fetal life. This can result in conditions like cervical hypoplasia, where the cervix is inherently smaller or weaker.
Impact of Past Pregnancies
A woman’s obstetric history is a major predictor for a short cervix in a current pregnancy. The single greatest risk factor is a prior spontaneous preterm birth, which suggests an underlying biological propensity for early cervical remodeling or cervical insufficiency. Cervical insufficiency means the cervix is unable to sustain the pregnancy to term. For women with this history, finding a short cervix in a subsequent pregnancy dramatically increases the risk of recurrence, sometimes to as high as 35 to 40 percent.
Procedures requiring mechanical dilation of the cervix can also decrease its structural competence. Dilation and curettage (D&C), used for managing miscarriage or termination, involves physically widening the cervical opening. This mechanical stretching, especially if difficult or repeated, can cause micro-trauma to the cervical muscle fibers and connective tissue. Forceful dilation is thought to weaken the internal cervical opening, leading to a decreased ability to maintain closure in later pregnancies.
Current Pregnancy Dynamics
Factors specific to the current gestation can trigger or exacerbate premature cervical shortening through both mechanical and biochemical pathways. When a pregnancy involves multiple gestations, such as twins or triplets, the cervix is subjected to significantly increased mechanical pressure and overall volume load. This chronic, elevated stress can lead to the premature effacement and shortening of the cervix.
Infection and inflammation are powerful drivers of cervical change, operating through a biochemical mechanism. Ascending infections, such as bacterial vaginosis, trigger an inflammatory response in the lower reproductive tract. This response involves the release of inflammatory mediators, including cytokines, which initiate cervical ripening. These mediators cause the breakdown of collagen and restructuring of the cervical matrix, signaling the cervix to prepare for labor much earlier than normal.
A genetic or hormonal predisposition can contribute to an inherently weaker cervix that is more susceptible to pressure. Conditions affecting connective tissue, like Ehlers-Danlos or Marfan syndrome, can result in cervical tissue with less tensile strength. Insufficient action of the hormone progesterone is also implicated in premature cervical change, as progesterone normally helps maintain the cervix in a long and firm state. This combination of weakness and hormonal sensitivity makes the cervix less resilient to mechanical and inflammatory stresses.