An incompetent cervix, now more commonly called cervical insufficiency, happens when the cervix opens too early during pregnancy, typically in the second trimester, without contractions or labor. It’s one of the leading causes of midtrimester pregnancy loss. The cervix, which normally stays firm and closed until late in pregnancy, weakens or shortens prematurely. A cervical length under 25 mm on transvaginal ultrasound is the standard threshold for diagnosing a short cervix at higher risk. The causes range from structural problems present from birth to damage acquired from surgery, infection, or the physical demands of pregnancy itself.
Prior Cervical Surgery
The most well-documented acquired cause is previous surgery on the cervix. Procedures like cone biopsy and loop excision (sometimes called LEEP), both used to treat abnormal cervical cells, remove tissue from the cervix. That lost tissue can permanently reduce the cervix’s ability to stay closed under the increasing weight of a growing pregnancy.
Research comparing pregnant women who had prior cone biopsy or LEEP to those without cervical surgery found significantly shorter cervical measurements in the surgical group: an average of 3.3 cm compared to 3.9 cm. Women in the surgical group also delivered slightly earlier on average, at 38.1 weeks versus 39.1 weeks, with higher rates of both late preterm and very preterm birth. The more tissue removed, the greater the potential impact on cervical strength.
Congenital Uterine and Cervical Abnormalities
Some women are born with structural differences in their reproductive tract that make the cervix inherently weaker. These result from incomplete development of the tubes that form the uterus, cervix, and upper vagina during fetal life. When those structures don’t develop, fuse, or reabsorb properly, the result can be a cervix that’s unusually small, malformed, or even absent.
The range of possible abnormalities is broad. A unicornuate uterus forms when only one side of the reproductive tract develops. A bicornuate or septate uterus results from incomplete fusion or failed reabsorption of the tissue dividing the uterine cavity. Some of these anomalies directly affect the cervix itself, creating a structure that simply can’t support a full-term pregnancy without intervention. Women with these conditions often don’t know about them until they experience pregnancy complications.
Connective Tissue Disorders
The cervix is largely made of collagen, the same structural protein that holds together skin, joints, and blood vessels. In conditions like Ehlers-Danlos syndrome, the body produces collagen that is structurally abnormal or deficient in key components. That same weakness that causes overly flexible joints or fragile skin also affects the cervix, making it more prone to opening under pressure.
Ehlers-Danlos syndrome encompasses at least ten distinct subtypes, each affecting connective tissue differently. In documented cases, women with EDS have experienced cervical insufficiency directly attributed to their underlying tissue fragility. The connection makes biological sense: if the collagen throughout the body is compromised, the cervix, which depends on collagen for its rigidity, will be compromised too.
Carrying Twins or Triplets
Multiple pregnancies place significantly more mechanical stress on the cervix than a singleton pregnancy. The extra weight and uterine stretching can overwhelm a cervix that might have held firm with just one baby. A large population-based study found that carrying multiples was one of the strongest risk factors for cervical insufficiency, with first-time mothers carrying multiples facing roughly eight times the risk compared to those with singleton pregnancies.
The risk compounds with other factors. Among first-time mothers with a history of prior miscarriage, carrying multiples raised the risk of cervical insufficiency dramatically. Women with one or more previous miscarriages and a multifetal pregnancy had up to 67 times the risk compared to those with singleton pregnancies and no miscarriage history. For women who had already given birth before, the independent effect of multiples was weaker, suggesting that a cervix that has successfully carried one pregnancy to term may be somewhat more resilient.
Infection and Inflammation
The cervix can weaken from the inside out. Subclinical infection or inflammation within the amniotic fluid triggers the release of enzymes that break down the cervical tissue, causing it to soften and shorten prematurely. This process mimics the natural ripening that happens at full term, but it occurs dangerously early.
What makes this cause particularly tricky is that it can happen without any obvious signs of infection. Sterile inflammation, where the body mounts an inflammatory response even without detectable bacteria, can produce the same cervical changes. Elevated levels of inflammatory markers and tissue-degrading enzymes have been found in the amniotic fluid of women with unexplained short cervixes. The body’s own immune activation, possibly triggered by internal “danger signals” rather than actual microbes, appears capable of initiating premature cervical opening.
DES Exposure
Diethylstilbestrol (DES) was a synthetic hormone prescribed to millions of pregnant women between the 1940s and 1970s to prevent miscarriage. It was later found to cause structural abnormalities in the reproductive tracts of daughters exposed in the womb. In one major study, structural anomalies of the cervix or vagina appeared in 25% to 49% of DES-exposed women, compared to just 2% of unexposed controls. The severity of these changes correlated with how early in pregnancy the exposure began and the total dose the mother received.
While DES is no longer prescribed, women born before the early 1970s whose mothers took the drug may still carry these structural changes. The resulting cervical abnormalities can include a smaller or irregularly shaped cervix that is more prone to insufficiency during pregnancy.
How Cervical Insufficiency Progresses
Regardless of the underlying cause, the physical process follows a recognizable pattern visible on ultrasound. A healthy mid-pregnancy cervix appears as a straight, closed channel, sometimes described as a T-shape. As it begins to weaken, the internal opening starts to widen while the external end stays closed, creating a Y-shape as the amniotic membranes begin pushing into the upper cervix. Further progression produces a V-shape, with the membranes funneling nearly to the outer opening, and finally a U-shape, which indicates advanced dilation. V- and U-shaped funneling are the most concerning findings, signaling that the cervix is well on its way to opening completely.
This progression can happen silently. Many women with cervical insufficiency feel no pain or contractions. The cervix simply opens, which is why routine cervical length screening by ultrasound between 16 and 24 weeks is recommended for women with known risk factors.
Treatment Options and Outcomes
The primary treatment for cervical insufficiency is cerclage, a stitch placed around the cervix to hold it closed. Two main techniques exist. One places the stitch slightly higher on the cervix and closer to the internal opening, while the other uses a simpler, lower placement. Research comparing the two approaches found that the higher placement reduced the risk of very early preterm birth (before 28 weeks) by about two-thirds and birth before 34 weeks by roughly half.
Timing of the cerclage matters less than many patients fear. A study comparing cerclage placed earlier in pregnancy to cerclage placed later found no significant difference in delivery timing: both groups delivered at a median of about 38 weeks. Newborn survival rates at three months were similarly high in both groups, around 95% to 98%. For women with a short cervix discovered on a routine scan, progesterone supplementation is another option that can help maintain cervical length and delay delivery, particularly in singleton pregnancies.
In many cases, the cause of cervical insufficiency is never definitively identified. It may result from a combination of subtle structural weakness, prior trauma, and the unique demands of a particular pregnancy. What matters most is early detection through cervical length monitoring, which gives clinicians time to intervene before the cervix opens too far.