The American College of Radiology has established a set of guidelines for the routine ultrasound examination to allow uniformity of expectations amoung clinicians across the country. This article pictorially reviews the standard features of a second and third-trimester ultrasound examination.
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
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
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
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
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
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
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
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,
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,