Obstetrics and Gynecology

Short cervix on ultrasound

Incidentally Discovered Short Cervix

1. What every clinician should know

Clinical features and incidence

Transvaginal ultrasound (TVU) cervical length (CL) is a screening test for prediction of preterm birth (PTB). A normal cervix is about 25-60 mm long outside of pregnancy and up to 28 weeks of gestation. After 28 weeks, even women destined to deliver at term (37 weeks or later) can begin to develop a short CL (less than 25 mm). A TVU CL ≤25 mm before 28 weeks is abnormal and associated with a high risk of PTB.

Short TVU CL can be detected by screening, or incidentally. TVU CL screening is recommended in women with singleton gestations and prior spontaneous PTB. In these women, a cerclage is recommended if TVU CL is ≤25 mm before 24 weeks. A short TVU CL can also be detected in women with singleton gestations but WITHOUT prior spontaneous PTB. This scenario is often called 'universal CL screening.' TVU CL can be used for screening multiple gestations, but this is not current recommended. If a short CL is detected, it is important to review the historic risk factors for PTB of the patient (see Table I).

Table I.

Risk factors for spontaneous preterm birth identifiable by history

Management varies, in particular depending on the history of a prior spontaneous PTB or not. Additionally, management differs if the current is a singleton or a multiple gestation. While there are efficacious interventions for women with a singleton gestation and a short CL, there are no such interventions for women with a short Cl and multiple gestations.

Risk factors

The presence of a short CL (≤25 mm) can be the final common pathway of many processes (see Figure 1).

Figure 1.

Pathways PTB

One of these is cervical insufficiency (CI), i.e. the presence of a weak cervix not strong enough to keep the conceptus in the uterus until the third trimester (see Cervical Insufficiency chapter). Several conditions can be associated with CI; in particular, significant prior cervical surgery. This would include multiple D&Es, major excisional cervical procedures (e.g. cold knife cone, multiple LEEPs, etc.) or other procedures on the cervix that can cause major trauma to its connective tissue (see CI chapter).

All risk factors for spontaneous PTB (Table I) should be discussed with the patient with an short CL. Symptoms of PTL should be elicited.

A speculum exam should be considered in women with a short TVU CL before 24 weeks, especially those with a TVU CL less than 15 mm. In these women, the cervix may appear visually open, with the possibility of fetal membranes prolapsing to a level past the internal os, and even past the external cervical os.

In women with prolapsed membranes and/or cervical dilatation not only of the internal but also of the external os, the risk of PTB is much higher. These women may be better served by cervical cerclage, while progesterone does not seem to be very effective at prevention of PTB once the cervix is open all the way from internal to external os.


Other conditions that can be associated with a short CL detected in the second trimester include inflammation, infection, contractions, uterine overdistension, etc. (Figure 1)

There are many associations (Figure 1) that can lead to an short cervix. In particular, prior cervical surgery and prior PTB are strong risk factors. Infection and inflammation play a pathophysiologic role in many cases as well.

As the incidence of intra-amniotic infection in most women (even those with prior PTB) with a short CL less than 25 mm detected before 24 weeks is less than 2%, an amniocentesis to routinely rule out infection is not recommended.

By American Congress of Obstetricians and Gynecologists (ACOG) and American Institute of Ultrasound in Medicine (AIUM) guidelines, every pregnant woman should have her lower uterine segment evaluated at the time of the second trimester (18-24 weeks) 'fetal anatomy survey' ultrasound because of the possibility of detecting a placenta previa. ACOG and the Society for Maternal-Fetal Medicine recommend TVU CL screening in women with singleton gestations and prior spontaneous PTB. These two societies deem 'reasonable' to screen with TVU CL also women with singleton gestations and without prior spontaneous PTB.

Short CL can only be diagnosed accurately with TVU. TAU is unreliable, poorly reproducible, and has poor sensitivity for detection of short CL and so should not be used for CL screening.

2. Diagnosis and differential diagnosis

Transvaginal ultrasound (TVU) cervical length (CL) is one of the best screening tests for prediction of preterm birth (PTB). Once a short CL is detected before 24 weeks, other tests that can be considered include: (a) gonorrhea, chlamydia, and other sexually-transmitted infections (STI), in particular in women at high risk for STIs; and (b) uterine tocodynamic monitoring to detect the presence of asymptomatic contractions.

Short CL should only be diagnosed by TVU. TAU should not be used to make this diagnosis. It is extremely important to use appropriate technique for TVU CL.

CL is the measurement to use for clinical management. Only one measurement should be reported by the sonologist to the clinician, i.e. the shortest best TVU CL. Other features of TVU of the cervix, such as funneling, are less reliable and valid for prediction of PTB and do not significantly add to the prognostic information provided by short CL alone. The presence of sludge, i.e. inflammatory material seen on TVU around the level of the internal os, increases the risk of PTB.

MRI or other radiologic techniques have not been shown to be equal or superior to TVU CL for prediction of PTB.

Tests such as fetal fibronectin (FFN) in asymptomatic women with a short CL have not been shown to improve perinatal outcome, and there are no laboratory studies that are helpful in making the diagnosis once a short CL is incidentally detected. Active research is being conducted on characteristics of the cervico-vaginal fluid, such as proteomics, genomics, inflammatory and infection markers and others.

Instead, the woman with short CL should be interviewed as per any risk factors for PTB. Additionally, symptoms of PTB should be elicited.

Interpretation of the short CL depends mostly on (a) the exact CL in cm; (b) the gestational age at which the CL is detected; (c) the presence (or not) of a prior PTB; and (d) the presence of a singleton vs. a multiple gestation.

3. Management

When a short TVU CL ≤25 mm is identified before 24 weeks, two interventions can be discussed with the patient. These are only effective in the woman carrying a singleton gestation. As there are no effective interventions to prevent PTB in multiple gestations with short CL, screening for short CL should not be performed in these pregnancies.

Singleton gestations without a prior spontaneous PTB and with a short TVU CL less than 21 mm before 25 weeks: randomized trials have shown that vaginal progesterone, either 200 mg or 90 mg gel, may prevent PTB.

Singleton gestations with a short CL less than 25 mm before 24 weeks and one or more prior PTBs: a large trial and then a meta-analysis of all trials in this population has shown that cerclage is associated with significant prevention of PTB (by 30%) and decreased composite perinatal morbidity and mortality (by 36%). Therefore, ultrasound-indicated cerclage should be recommended in this clinical scenario and performed as soon as feasible, usually within 24-48 hours.

In these women, 17-alpha hydroxy progesterone 250 mg IM should have been started at 16 weeks and weekly thereafter based on the prior spontaneous PTB. If this has not been done earlier, it should be again recommended and implemented.

Other therapies such as pessary, bed rest, antibiotics, etc., have not been consistently shown to reduce complications.

Indomethacin has been used in one very effective trial in conjunction with an ultrasound-indicated cerclage in women with both prior spontaneous PTB and a short CL less than 25 mm and may be considered. If used, the dose is 100 mg loading dose per rectum or vagina, and then 50 mg every 6 hours. it should be stopped after 48 hours to avoid fetal harm.

4. Complications

There are many common pitfalls to the identification of a short CL. This can be easily avoided by adhering to proper TVU CL technique.

In women with short CL with singleton gestation and no risk factors for PTB (e.g. prior PTB), the chance of PTB may be as low as 10-20%, especially if the CL is 21-24 mm. In these scenarios, it is reasonable to repeat a TVU CL in 1-2 weeks to see if the CL becomes shorter than 21 mm, and therefore the patient becomes eligible for vaginal progesterone therapy. In many cases, the CL remains greater than 20 mm until after 24 weeks.

5. Prognosis and outcome

Prognosis depends mostly on (a) the exact CL in cm; (b) the gestational age at which the CL is detected; (c) the presence (or not) of a prior PTB; and (d) the presence of a singleton vs. a multiple gestation.

In singleton gestations without prior spontaneous PTB and other significant risk factors for PTB, a short CL less than 25 mm before 24 weeks is associated with an approximate 20-30% risk of PTB before 35 weeks.

In singleton gestations with prior PTB, a short CL less than 25 mm before 24 weeks is associated with an approximate 4% or more risk of PTB before 35 weeks. The shorter the CL and the earlier the GA at detection, the higher is the risk of PTB.

6. What is the evidence for specific management and treatment recommendations

Berghella, V.. Preterm birth: Prevention and Management. Wiley-Blackwell. 2010.

(This is an evidence based medicine book focused on the impact, epidemiology, genetics and prevention of preterm birth.)

"Assessment of risk factors for preterm birth". ACOG Practice Bulletin. 2001.

The guidelines set as current standard of care by the US ACOG group.

Iams, JD, Berghella, V.. "Care for women with prior PTB". Am J Obstet Gynecol. 2010. pp. 89-100.

Meis, PJ, Klebanoff, M, Thom, E. "Prevention of recurrent preterm delivery by 17 alphahydroxyprogesterone caproate". N Engl J Med. vol. 348. 2003. pp. 2379-86.

(Randomized controlled trial comparing the use of 17P versus placebo in women with a history of prior preterm birth. The evidence showed that intramuscular administration of 17P can decrease the risk of recurrent preterm birth when started between 16-20 weeks and continued weekly until 36 weeks.)

"Use of progesterone to reduce preterm birth. ACOG Committee Opinion No.419". Obstet Gynecol. vol. 112. 2008. pp. 963-5.

(An ACOG committee opinion regarding the Meis trial above.)

Berghella, V, Rafael, TJ, Szychowski, JM, Rust, OA, Owen, J.. "Cerclage for short cervix on ultrasonography in women with singleton gestations and previous preterm birth: a meta-analysis". Obstet Gynecol. vol. 117. 2011. pp. 663-71.

(A meta-analysis of previous trials that studied the outcomes after the placement of cerclage when short cervix was diagnosed on ultrasound in women who had a previous preterm birth.)

Berghella, V, Obido, AO, To, MS, Rust, OA, Althiusius, SM.. "Cerclage for short cervix on ultrasound: Meta-analysis of trials using individual patient-level data". Obstet Gynecol. vol. 106. 2005. pp. 181-9.

Fonseca, EB, Celik, E, Parra, M, Singh, M, Nicolaides, KH.. "Progesterone and the risk of preterm birth among women with a short cervix". N Eng J Med. vol. 357. 2007. pp. 462-9.

Hassan, SS, Romero, R, Vidyadhari, D, Fusey, S, Baxter, J. "for the PREGNANT Trial". Ultrasound Obstet Gynecol. 2011 Apr 6.

(This study focused on cervical length screening in a low risk population and subsequent treatment with vaginal progesterone for women found to have a short CL.)

Cahill, AG, Odibo, AO, Caughey, AB, Stamilio, DM, Hassan, SS. "Universal cervical length screening and treatment with vaginal progesterone to prevent preterm birth: a decision and economic analysis". Am J Obstet Gynecol. vol. 202. 2010. pp. 548.e1-8.

(An economic analysis of the impact of screening all low risk women for a short cervix.)

Werner, EF, Han, CS, Pettker, CM, Buhimschi, CS, Copel, JA. "Universal cervical-length screening to prevent preterm birth: a cost-effectiveness analysis". Ultrasound Obstet Gynecol. vol. 38. 2011. pp. 32-7.

Iams, JD, Goldenberg, RL, Meis, PJ, Mercer, BM, Moawad, A. "The length of the cervix and the risk of spontaneous premature delivery". NEJM. vol. 334. 1996. pp. 567-72.

Berghella, V, Rust, OA, Althuisius, SM.. "Short cervix on ultrasound: does indomethacin prevent preterm birth?". Am J Obstet Gynecol. vol. 195. 2006. pp. 809-13.

DeFranco, EA, O’Brien, JM, Adair, CD. "Vaginal progesterone is associated with a decrease in risk for early preterm birth and improved neonatal outcome in women with a short cervix: a secondary analysis from a randomized, double-blind, placebo-controlled trial". Obstet Gynecol. vol. 30. 2007. pp. 697-705.

Owen, J, Szychowski, J.. "Can the optimal cervical length for placing ultrasound-indicated cerclage be identified?". Am J Obstet Gynecol. vol. 204. 2011. pp. s198-9.

Berghella, V, Keeler, SM, To, MS. "Effectiveness of cerclage according to severity of cervical length shortening: a meta-analysis". Ultrasound Obstet Gynecol. vol. 35. 2010. pp. 468-73.

Althuisius, SM, Dekker, GA, Hummel, P, van Geijn, HP.. "Cervical incompetence prevention randomized cerclage trial. Emergency cerclage with bed rest versus bed rest alone". Am J Obstet Gynecol. vol. 189. 2003. pp. 907.

Durnwald, CP, Momirova, V, Rouse, DJ. "For the Eunice Kennedy Shriver National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network. Second trimester cervical length and risk of preterm birth in women with twin gestations treated with 17-α hydroxyprogesterone caproate". J Matern Fetal Neonat Med. 2010. pp. e1-5.

SMFM, Berghella. "Progesterone and preterm birth prevention: translating clinical trials data into clinical practice". Am J Obstet Gynecol.. vol. 206. 2012 May. pp. 376-86.

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