Applied Radiology

The American College of Radiology has established a set of guidelines for the routine obstetrical ultrasound examination.1 These allow for uniformity of expectations among clinicians across the country, and has eliminated much of the confusion concerning various "levels" of exams (i.e. level 1 vs 2 ultrasound). This article serves to illustrate the standard features of a second- or third-trimester ultrasound exam.

The initial portion of the exam should determine the number of fetuses pres-

ent, as well as the presentation (cephalic, breech, transverse) and activity (cardiac and trunk/extremity motion) of the fetus. At this point in the exam, the uterine wall and adnexae can quickly be evaluated to determine if adnexal masses, uterine leiomyomas, or other developmental features are present, realizing that normal ovaries may not be demonstrated at this stage of pregnancy.

The amniotic fluid volume should be characterized as low (figure 1), normal, or increased (figure 2). If subjective assessment is abnormal, a semi-quantitative measurement known as the amniotic fluid index can be calculated. This is obtained by imaging the four "quadrants" of the uterus in a transverse plane and adding the anteroposterior diameters (depths) of fluid in each location, being careful not to include fetal body parts or the umbilical cord in the determination. Although this measurement varies throughout a pregnancy, the normal values are approximately 8 to 24 during the second and third trimesters.

Placental location in relationship to the cervical os must be evaluated on all sonography exams, thought it is of greater significance later in gestation. Most placenta previae found before 28-weeks' gestation will resolve by term; however, follow-up limited exams can be performed prior to delivery (figure 3). Transperineal imaging is one simple technique that allows for improved visualization of the cervical os and placenta in cases where placenta previa is suspected. A 3.5 MHz transducer can be gloved and placed on the maternal peri-

neum external to the vagina for this portion of the exam (figures 4 and 5).

Gestational age charts exist for many different fetal-part measurements. Standard exams should include a biparietal diameter (figure 6) or preferably a head circumference or a corrected biparietal diameter (figure 7), which will take into account the shape of the fetal head as well as femur length (figure 8) in assessing gestational age. The abdominal circumference is then added to determine fetal weight and to calculate a head-to-abdominal circumference ratio to assess fetal growth. Comparing previous exam results to current findings can help to determine appropriate interval fetal growth. Of course, the appropriate images are essential for accurate assessment of gestational age and fetal growth (figures 9A and 9B).

Fetal structures on examination

The fetal "review of systems" serves to exclude many, but not all, fetal malformations. However, documentation of the following fetal structures is considered the standard of care at the present time. If visualization of some of these structures is limited or not possible upon examination, the report should reflect the omissions.

Complete evaluation of the neural axis will include imaging of the cerebral ventricles, posterior fossa, and the spine. The lateral ventricular atria should measure less than 10 mm in diameter from 15 weeks of gestation to term, thereby excluding hydrocephalus (figure 10). This measurement should be made just above the level used to measure the biparietal diameter (i.e. just superior to the thalamus). The posterior fossa can be imaged transaxially, just superior to the base of the skull, and the view should include the cerebellar hemispheres and cisterna magna (figure 11).

The anteroposterior depth of the cisterna magna in the midline measures 2 mm to 11 mm in the second and third trimesters (figure 12). Effacement of the cisterna magna suggests an Arnold-Chiari II malformation with associated meningocele, while a normal appearance of the posterior fossa virtually excludes the possibility of spina bifida; all this before the spine is even imaged! The spine itself should be scanned in longitudinal and transverse planes (images of the entire spine in the coronal or sagittal plane should be accompanied by a few representative transverse images for the permanent record) (figures 13A, 13B, 13C, and 14).

With a coronal view, the face can be evaluated for midline facial defects such as cleft palate, although this is not yet considered a part of the standard exam (figures 15 and 16). A profile view may also provide a clue to fetal anomalies, including a small jaw (micrognathia), flattened nose, etc. (figure 17).

A four-chamber view of the fetal heart (figure 18) is essential and, while not yet considered standard, the left ventricular outflow tract view is helpful in excluding additional cardiac defects such as tetralogy of Fallot, truncus arteriosus, and transposition of the great vessels. This view can be obtained by rotating the transducer from the transverse four-chamber view toward the fetal right shoulder (figure 19).

The fetal stomach must be visualized in the proper left upper quadrant location (figure 20), with the umbilical cord insertion site documented to assure an intact abdominal wall (figure 21). Presence of a three-vessel cord (two arteries and one vein) should be also established (figure 22).

In all second- and third-trimester fetuses, the kidneys and bladder must

be analyzed to exclude genitourinary anomalies. The kidneys are most easily seen on a transaxial plane as relatively hypoechoic structures in a paraspinal location (figure 23A and 23B). The bladder is low and is located anteriorly in the pelvis (figure 24).

Additional considerations

Parents will often want to know the gender of their child, which is not required in the minimal standard exam. With a good view of the fetal perineum, however, gender can reliably be ascertained from approximately 15 weeks to term (figures 25A and 25B).

A more-extensive fetal ultrasound exam also may be required in certain clinical situations, including the history of a previous fetal anomaly, elevated or decreased maternal serum alphafetaprotein (MS-AFP), increased or decreased amniotic fluid levels, and the presence of a structural fetal anomaly. However, a routine exam consisting of the above-mentioned features is considered a current standard of care. When a more-detailed fetal anatomical survey is deemed necessary, appropriate referral to specialists in prenatal ultrasound is indicated.

This routine obstetrical ultrasound exam, including documentation, should be attainable in twenty to thirty minutes. Current ultrasonography equipment includes obstetrical packages which calculate gestational age and various ratios, as well as the estimated date of delivery based on the ultrasound exam, all of which is helpful to the obstetrician and sonologist for follow-up exams.

As there are currently so many individuals performing obstetrical ultrasounds, each with differing qualifications, utilization of the ACR guidelines in routine examinations will ensure a quality exam for the patients and improved communication for clinicians. Happy scanning (figure 26)! AR

References

1. American College of Radiology: Standard for the performance of antepartum obstetrical ultrasound, 1990, revised 1995.

2. England MA: A colour atlas of life before birth. Normal fetal development. England: Wolfe Medical Publications Ltd., 1983, 1990.

3. Bowerman RA: Atlas of normal fetal ultrasonographic anatomy. Chicago, Year Book Medical Publishers, 1986.

4. Callen PW: Ultrasonography in obstetrics and gynecology, ed 3. Philadelphia, WB Saunders, 1994.

5. Rumack CM, Wilson SR, Charboneau JW (eds): Diagnostic ultrasound, St. Louis, Mosby-Year book, 1991.

6. Hertzberg BS: Cervical disease and transperineal imaging. Ultrasound and women's health course syllabus. Baltimore, Maryland, ALUM, 1994.