Ultrasound Evaluation of the Gravid Cervix

Transabdominal Ultrasound

In the 1970s, the first attempts at evaluating the cervix used trans­abdominal ultrasound (TAU). Unfortunately, this technique has relatively poor reliability and validity because of the following shortcomings: (1) the bladder often needs to be adequately filled to obtain a good image, resulting in elongation of the cervix and masking of any funneling of the internal os; (2) fetal parts can obscure the cervix, especially after 20 weeks; (3) the distance from the probe to the cervix results in degraded image quality; and (4) obesity and manual pressure interfere with the resulting image. Therefore, it is generally suggested that TAU should not be used for assessment of the cervix, even as a screening test, because its sensitivity for prediction of disease (e.g., increased risk for PTB) is unacceptably low (8%) in comparison to TVU. All randomized controlled trials (RCTs) that proved benefit with different treatments for a sonographically detected short cervix used TVU: vaginal progesterone, cerclage, and pessary. The sensitivity of TVU has been shown to be significantly higher than TAU in detecting a short cervix ( Table 18-1 ).

As can be seen from the studies described in Table 18-1 , it is still unclear if TAU is better done before or after voiding; only two studies report blind results. The cutoff for TAU-detected CL measurement needs to be high (usually >35 mm) to achieve a sensitivity of more than 90% for a shortened cervix by TVU, in which case most patients screened with TAU would need a subsequent TVU to confirm or rule out a short cervix. In 9% to 51% of attempted TAU, CL was not obtainable. Moreover, TVU CL screening has been shown to have superior cost-effectiveness compared to TAU, with TVU associated with slightly better prevention of PTB, resulting in cost savings of millions of dollars. All major guidelines that have described CL screening have clearly recommended TVU, including the Society for Maternal-Fetal Medicine (SMFM), the American College of Obstetricians and Gynecologists (ACOG), and the Royal College of Obstetricians and Gynaecologists. Given the preponderance of the evidence, TAU is less effective than TVU and should not be used for CL screening.

Translabial Ultrasound

Translabial (also known as transperineal) ultrasound (TLU) was first described in France in the early 1980s. This technique involves having the woman lie on the examination table with her hips and knees flexed. Adjusting the examination table to place her in a Trendelenburg position can be helpful. A covered transducer (using a probe cover, or glove, and sterile gel) is positioned in a sagittal orientation on the perineum between the labia majora at the level of the vaginal introitus ( Fig. 18-1 ). Elevation of the woman's hips with a cushion or by positioning her hands to achieve a pelvic tilt can be used to improve visualization. Unlike TAU, this technique does not require bladder filling, is not impaired by obscuration by fetal parts, and with the transducer closer to the cervix, achieves close to 100% visualization. Other advantages of this technique are that the transducer does not enter the vagina (so no pressure can be exerted on the cervix thus no alteration in appearance from mechanical effect), it does not require an additional transducer, it can be performed in the setting of ruptured membranes, and it is well accepted by most women. The main drawback of TLU, however, is that gas in the rectum sometimes impedes visualization of the cervix, especially the external os. TLU is also more difficult to master than TVU and has been reported to be an unsatisfactory alternative. However, with experience in using TLU, adequate visualization of the cervix can often be obtained.

Transvaginal Ultrasound

The first studies of the human cervix using TVU date back to the late 1980s. A clean transvaginal probe covered by a condom is gently inserted into the vagina ( Fig. 18-2A and B ). The technique shares the advantages of TLU; however, the probe is close to the cervix, without the problem of obscuring bowel gas. Thus, TVU has become the preferred approach for sonographic evaluation of the gravid cervix in virtually all clinical settings ( Fig. 18-3A to C ). Current recommendations for the performance of TVU of the cervix are shown in Table 18-2 .

The total time for TVU of the cervix is approximately 5 minutes. For optimal results, the following criteria should be sought: the internal os should be either flat or at an isosceles angle with respect to the uterus, the whole length of the cervix should be visualized, a symmetric image of the external os should be obtained, the distance from the surface of the posterior lip to the cervical canal should be equal to the distance from the surface of the anterior lip to the cervical canal, and there should be no increased echogenicity in the cervix (a sign of excessive pressure).

Successful completion of the education program, examination, and continuing image review, all provided by the Perinatal Quality Foundation of the SMFM, is available and is recommended. Ideally, TVU CL assessment should only be performed after this program has been completed.

Full Bladder

A bladder that is not completely empty of urine can exert pressure on and appear to elongate the cervix or might mask funneling or opening of the internal os.

Excessive Pressure

Excessive pressure by the examiner with the transvaginal transducer can also mask funneling or opening of the internal os and can elongate the cervix. This can be recognized by alterations in appearance including asymmetry in thickness of the anterior and posterior walls of the cervical canal and by increased echogenicity of the cervix ( Fig. 18-4 ).

Contraction

Uterine segment contractions may mimic the appearance of funneling of the internal cervical os. In such cases, there is rounded myometrium around the cervix and a normal cervix distal to the contraction ( Fig. 18-5 ).

Underdeveloped Lower Uterine Segment

Before 14 weeks, the lower uterine segment is difficult to distinguish from the endocervical canal, because the gestational sac has not reached a sufficient size to completely expand the lower part of the uterus. Therefore, the measurement of the true CL is difficult before 14 weeks. At other times, particularly between 14 and 18 weeks' gestation, the presence of a contraction can close the lower uterine segment and make the distinction between this structure and the true CL difficult.

Transvaginal Ultrasound of the Cervix for Prediction of Preterm Birth