The uterus is a pear-shaped organ made up of the fundus, body and cervix. The cervix consists of three parts: internal os, cervical canal and external os. This uterine cavity communicates via the internal os with the cervical canal which, in turn, opens into the vagina by the external os.

In fetal life the cervix is considerably larger than the body; in childhood (the infantile uterus) the cervix is still twice the size of the body but, during puberty, the uterus enlarges to its adult size.

 Cervical positioning

•  Flexion describes positioning of uterine body relative to cervix

 Most uteri are anteflexed

•  Version describes axis of cervix relative to vagina

 Most uteri are anteverted

An anteflexed uterus is a uterus with an acute or obtuse angle (< 180 degrees) between the cervix/lower uterine (isthmus) segment and the fundus with the fundal portion close to the bladder. A retroflexed uterus is a uterus with a reflex angle (> 180 degrees) between the cervix/lower uterine (isthmus) segment and the fundus with the fundal portion close to the rectosigmoid

The most common position of the uterus is anteverted (cervix angles forward) and anteflexed (body is flexed forward). However, the position of the uterus in the adult is liable to considerable variation.

AV AF: anteverted anteflexed

AV: anteverted

AV RF: anteverted retroflexed

Evaluation of the cervix is typically performed by using standard transabdominal or transvaginal probe. The transabdominal approach pro­vides a global overview of the anatomy but may not allow visualization of the cervix in 15%–55% of patients (1).  The transvaginal approach usually results in more optimal visualization of the cervix because of the ability to place the transducer closer to the cervix and to use higher-frequency probes (2,3)

In addition to these techniques, transrectal or transperineal approach can be used if transvaginal imaging is not possible (e.g., premenarche). Transperineal may provide superior imaging in up to 75% of patients compared with transabdominal examination (4).

According to Boelig et al, Fifteen percent of trained imagers failed to obtain appropriate cervical length imaging. This highlights the importance of a standardized cervical length training and certification program. (5)

In the transabdominal approach, patient lies flat with placing the probe suprapubic and the bladder is partially full. On the other hand, during transvaginal or transrectal routes patient lies in the dorsal lithotomy position with the bladder empty.

The following factors need to be taken into account:

• probes should be covered with a single-use barrier such as surgical gloves or condom. If condoms are used as barriers, they should be nonlubricated and nonmedicated. For maximum safety, one should therefore perform high-level disinfection of the probe between each use and use a probe cover such as surgical gloves or condom as an aid to keep the probe clean.
• Maternal bladder should be empty (during tranvaginal or transrectal approach).
• A longitudinal view of the cervix should be obtained (sagittal view).
• The cervical canal and surrounding cervical mucosa should be identified.
• Magnification of the ultrasound image needs to be appropriate. The cervix should occupy approximately 50–75% of the image.
• Pressure from the probe on the cervix should be as little as possible to assess cervical mobility.
• Calipers should be placed correctly (Measurement of the cervical length can be done either by straight line from the internal os to external os or by line trace method)

Color Doppler US may also help to define the vascular supply of a mass eg; polyp or fibroid that has been reported to be helpful in detection and characterization of endometrial and uterine abnormalities (6-8). Saline-infused sonohysterography also can be used to aid in detection and characterization of intra­cavitary and endocervical lesions (9).

The external os is identified as the point at which the anterior and posterior lips of the cervix come together. Identification of the location of the internal os can be more challenging. In order to do so, the cervical mucosa must be identified. A thin line of demarcation between the stroma and the cervical mucosa can generally be identified on ultrasound. The point at which the cervical mucosa ends is considered to be the internal cervical os. (10)

Indications of cervical length measurement

It has been shown that a shortened cervix is a powerful indicator of preterm births in women with singleton and twin gestations – the shorter the cervical length, the higher the risk of spontaneous preterm birth. (11), the cervix should be assessed between 16 and 24 weeks gestation.

According to Kipros Nicolaides and the Fetal Medical Foundation, standards have been established for measuring cervical length in the mid-trimester (12). 

Cut off cervical length

The bulk of the evidence for short cervical length and risk of preterm birth is from studies using a single cut-off of either 20 or 25 mm between 18 and 24 weeks gestation (13).

A cervical length ≤ 25 mm at 18 weeks (LR+ 9.7, sensitivity 14.3%) and ≤ 22 mm at 24 weeks (LR+ 9.6, sensitivity 28.6%) were the best predictors of preterm delivery in twins (14).

Dynamic cervical change, or shortening of the cervix during real‐time ultrasound, has been described (15).

U/S is an excellent modality for evaluation of müllerian anomalies, including anomalies of the cervix, particularly the presence of a septum, duplication, or agenesis.In the case of a complete uterine septum, the cervical canal is divided by a plane of tissue that may appear hypoechoic if fibrous. (16)

1- Cervicitis: At U/S, the cervix shows a diffusely heterogeneous echotexture of the cervical mucosa and stroma, but the key finding is markedly increased vascularity (16)

2- Nabothian Cysts: Nabothian cysts are the most common benign mass-like lesions of the cervix. Most nabothian cysts appear as simple an-echoic cysts in the cervix, without vascularity. (16)

3- Endocervical Polyps: At gray-scale U/S, endocervical polyps typically appear slightly hyperechoic compared with normal mucosa and may be mobile at dynamic imaging with use of transducer pressure. Color and spectral Doppler US may reveal a vascular stalk arising from the endocervical mucosa and extending into the polyp, confirming the endocervical origin. (16)

4- Leiomyomas: At U/S, cervical leiomyomas are typically solid, hypoechoic, and centered in the cervical stroma and may demonstrate refractory shadowing. (16)

^{1. Scheerer LJ, Bartolucci L. Ultrasound evaluation of the cervix. In: Callen PW, ed. Ultrasonography in obstetrics and gynecology. 5th ed. Philadelphia, Pa: Saunders, 2007; 479–485.}

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^{3. American Institute of Ultrasound in Medicine (AIUM); American College of Radiology (ACR); American Col¬lege of Obstetricians and Gynecologists (ACOG); Society for Pediatric Radiology (SPR); Society of Radiologists in Ultrasound (SRU). AIUM practice guideline for the per¬formance of ultrasound of the female pelvis. J Ultrasound Med 2014;33(6):1122–1130.}

^{4. Mahoney BS. Translabial and transvaginal ultrasound. In: Nyberg DA, Hill LM, Bohm-Velez M, Mendelson EB, eds. Transvaginal ultrasound. St Louis, Mo: Mosby Year Book, 1992; 169–186.}

^{5. Rupsa C Boelig , Helen Feltovich, Jean Lea Spitz, Gregory Toland, Vincenzo Berghella, Jay D Iams. Assessment of Transvaginal Ultrasound Cervical Length Image Quality.Obstet Gynecol 2017; 129:1-7.}

^{6. Fleischer AC. Color Doppler sonography of uterine disorders. Ultrasound Q 2003;19(4):179–189.}

^{7. Timmerman D, Verguts J, Konstantinovic ML, et al. The pedicle artery sign based on sonography with color Dop¬pler imaging can replace second-stage tests in women with abnormal vaginal bleeding. Ultrasound Obstet Gynecol 2003;22(2):166–171.}

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^{9. Davis PC, O’Neill MJ, Yoder IC, Lee SI, Mueller PR. Sono¬hysterographic findings of endometrial and subendometrial conditions. RadioGraphics 2002;22(4):803–816.}

^{10. K. O. Kagan and J. Sonex. How to measure the cervix. Ultrasound Obstet Gynecol 2015; 45: 358–362}

^{11. O'Hara S, Zelesco M, Sun Z Cervical length for predicting preterm birth and a comparison of ultrasonic measurement techniques. Australas J Ultrasound Med. 2013 Aug;16(3):124-134.}

^{12. Celik E, To M, Gajewska K, Smith GC, Nicolaides KH; Fetal Medicine Foundation Second Trimester Screening Group. Cervical length and obstetric history predict spontaneous preterm birth: development and validation of a model to provide individualized risk assessment. Ultrasound Obstet Gynecol 2008;31:549-54.}

^{13. Hibbard JU, Tart M, Moaward AH. Cervical length at 16-22 weeks’ gestation and risk for preterm delivery. Obstet Gynecol 2000;96(6):972-8.}

^{14. J.L. Gibson, L. M. Macara, P. Owen, D. Young, J. Macauley and F. Mackenzie. Prediction of preterm delivery in twin pregnancy: a prospective, observational study of cervical length and fetal fibronectin testing. Ultrasound Obstet Gynecol 2004; 23: 561–566}

^{15- J.T.Kurtzman , S.M.Jenkins , W.R.Brewster. Dynamic cervical change during real‐time ultrasound: prospective characterization and comparison in patients with and without symptoms of preterm labor. Ultrasound Obstet Gynecol 2004; 23: 574–578}

^{16. Wildenberg JC, Yam BL, Langer JE, Jones LP. Radiographics. US of the Nongravid Cervix with Multimodality Imaging Correlation: Normal Appearance, Pathologic Conditions, and Diagnostic Pitfalls. 2016 Mar-Apr;36(2):596-617.}