Visualization of cervical shortening on real-time imaging, with or without fundal or suprapubic pressure, raises preterm birth risks.
Shortest cervical length image that meets all criteria should be used.
Modified from lams, 2013.
by technical factors that include maternal habitus, cervical position, or shadowing by the fetal presenting part. In addition, the maternal bladder or pressure from the transducer may artificially elongate the appearance of the cervix. As a result, values from transabdominal or transvaginal measurement of the cervix can difer significantly.
If the cervix appears shortened or if it cannot be adequately visualized during transabdominal evaluation, transvaginal assessment is considered (American Institute of Ultrasound in Medicine, 2013b). Only cervical length measurements obtained transvaginally at or beyond 16 weeks' gestation are considered sufficiently accurate for clinical decision-making (see Fig. 10-4). A foreshortened cervix is associated with an elevated risk for preterm birth, particularly in the setting of prior preterm birth, and the degree of risk rises proportionally with the degree of cervical shortening (Chap. 42, p. 815).
To measure the cervix transvaginally, the imaging criteria shown in Table 10-8 are followed. he endocervical canal should be visible in its entirety, and images ideally are obtained over several minutes to allow for dynamic change. During examination, visible funneling or debris is sought. Funneling is a protrusion of amnionic membranes into a portion of the endocervical canal that has dilated (Fig. 10-5). Funneling is not an independent predictor of preterm birth, however, it is associated with cervical shortening, and transvaginal assessment is recommended if a funnel is suspected transabdominally. The cervical length is measured distal to the funnel, because the base of the funnel becomes the functional internal os. If the cervix is dilated, as with cervical insuiciency, the membranes may prolapse through the endocervical canal and into the vagina, producing an
FIGURE 10-5 Transvaginal image depicting a foreshortened cervix with funneling. Funneling is a protrusion of amnionic membranes into a portion of the endocervical canal that has dilated. The distal protruding edge of the funnel becomes the functional internal os (left arrow). Thus, the measured cervical length, which lies between the arrows, should not include the funnel. (Used with permission from Dr. Emily Adhikari.)
FIGURE 10-7 Transcerebellar view of the posterior fossa, demonstrating measurement of the cerebellum (+), cisterna magna (x), and nuchal fold thickness (backet). Care is taken not to angle obliquely down the spine, which may artificially increase the nuchal fold measurement.
FIGURE 10-6 The transventricular view depicts the lateral ventricles, which contain the echogenic choroid plexus (CP). The lateral ventricle is measured at the atrium (arrows), which is the confluence of the temporal and occipital horns. A normal measurement is between 5 and 10 mm throughout the second and third trimesters. The atria measured 6 mm in this 21-week fetus.
hourglass appearance. Sludge or debris represents an aggregate of particulate matter within the amnionic sac, close to the internal os. In pregnancies at risk for preterm birth, sludge is associated with a further increased risk.
Many fetal anomalies and syndromes may be characterized with targeted sonography, and selected abnormalities are discussed subsequently. This list is not intended to be comprehensive but covers abnormalities commonly detected with standard sonography and those that are potentially amenable to fetal therapy. Sonographic features of chromosomal abnormalities are reviewed in Chapters 13 and 14, and fetal therapy is discussed in Chapter 16.