Targeted Obstetric Ultrasound

Updated: Apr 28, 2021
  • Author: Charles Egan, DO; Chief Editor: Carl V Smith, MD  more...
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Introduction to Targeted Obstetric Ultrasonography

Obstetric ultrasonography is a frequent consult in radiology, whether it be early in pregnancy to evaluate intrauterine gestation, at mid-term to evaluate fetal anatomy, or at near-term when issues of fetal growth are at hand. Technological advances in ultrasonography such as 3-dimensional and 4-dimensional ultrasonography are in high demand, both from the public and the general medical community. [1] Concern for the viability of pregnancy, fetal anomalies, and fetal well-being are commonly evaluated with obstetric ultrasonography.

For myriad reasons, it is beneficial to have a basic understanding of the targeted obstetric ultrasound examination. Questions regarding obstetric ultrasound indications, safety, and implications of imaging findings may come up in the course of daily practice. The purpose of this article is to bring to light these issues and to discuss their implications.


Indications for Obstetric Ultrasound

Some of the acceptable indications for ultrasound evaluation in the first trimester are as follows [2] :

  • Vaginal bleeding, pelvic pain, or any concern for an ectopic pregnancy.

  • To confirm intrauterine gestation and cardiac activity and to estimate gestational age.

  • To evaluate pelvic or uterine masses.

  • To assess for certain fetal anomalies in high-risk patients, including anencephaly.

  • Concern for molar pregnancy.

  • As an adjunct to chorionic villus sampling, embryo transfer, or localization and removal of an intrauterine device.

  • To measure nuchal translucency as part of the evaluation for chromosomal aneuploidy.

  • The CDC and ACOG recommend that pregnant women who live in or have traveled to areas with ongoing Zika virus exposure should undergo Zika virus serologic testing and fetal ultrasonography to screen for microcephaly or intracranial calcifications as early as 3-4 weeks after symptoms or exposure. [3, 4] However, the CDC warned that fetal ultrasounds might not detect abnormalities until late second or early third trimester of pregnancy. [5, 6, 3]

  • Guidelines from the International Society of Ultrasound in Obstetrics and Gynecology recommend that pregnant women with suspected or probable coronavirus disease (COVID-19), or those with confirmed infection who are asymptomatic or recovering from mild illness, should be monitored with 2–4-weekly ultrasound assessment of fetal growth and amniotic fluid volume, with umbilical artery Doppler if necessary. [7]

Some acceptable indications for ultrasound evaluation in the second or third trimester are listed below [2] :

  • To screen for fetal anomalies or to determine fetal well-being [8]

  • To evaluate fetal growth and/or estimate gestational age [9]

  • To determine fetal presentation

  • Follow-up evaluations for a fetal or placental anomaly

  • To evaluate premature rupture of membranes or preterm labor

  • To evaluate abnormal biochemical markers

  • To evaluate a suspected placental abruption

  • Adjunct to amniocentesis, cervical cerclage placement, external cephalic version, or other procedure

  • To look for findings that may increase the risk for aneuploidy (eg, nuchal thickening, echogenic bowel, two-vessel cord)

  • To evaluate a patient with a history of a congenital abnormality

  • To evaluate suspected fetal demise


Fetal Safety

Ultrasonography uses mechanical radiation in the form of sound waves to produce images. This differs from the ionizing radiation involved in CT scanning and plain film radiography. However, any radiation poses potential risks.

The bioeffects of ultrasound are based on two variables, called the mechanical index and the thermal index (see image below).

Thermal index (TI) and mechanical index (MI) measu Thermal index (TI) and mechanical index (MI) measurements obtained during a routine obstetric ultrasound examination.

The mechanical index describes ultrasound's ability to cause air bubbles to rise and collapse, theoretically causing a rapid increase in temperature. However, this bioeffect is negligible because the fetal bowel and lungs are airless en utero.

The thermal index is founded on the heating principle: Sound waves deposit energy into the soft tissues, and this is converted to heat. The thermal index is increased with prolonged static examinations and Doppler ultrasound. In the hands of a practitioner who is not adequately trained to properly perform an ultrasound examination, this has the potential for harm, especially in the first trimester. [10]

Despite the theoretical potential for harm, no data have shown any significant detrimental effect to the fetus from a routine obstetric ultrasound examination. Some epidemiological studies have suggested adverse effects, such as growth restriction or delayed speech, but these have not been statistically proven. [11] Nonetheless, it is important for physicians and technicians alike to be mindful of these potential complications and to continue to practice the ALARA (as low as reasonably achievable) principle when conducting examinations.


First-Trimester Ultrasound Evaluation

In the first trimester, identifying an intra-uterine gestational sac is a top priority. A transvaginal or transabdominal approach is acceptable. However, if the findings of the transabdominal ultrasound are not definitive (eg, nonvisualized gestational sac), a transvaginal examination is recommended, as it provides higher sensitivity.

A gestational sac can be seen as early as the fourth menstrual week with the transvaginal approach. The "double decidual" sign, which correlates with the decidual reaction that occurs following implantation, is seen in 85% of pregnancies. [12] Of course, the presence of a yolk sac or fetal pole confirms the presence of a gestational sac (see images below).

Coronal transvaginal image through the uterus demo Coronal transvaginal image through the uterus demonstrates a gestational sac with a normal surrounding decidual reaction. Note the presence of a normal-appearing yolk sac (arrow) within the gestational sac.
Normal fetal pole within a gestational sac. Normal fetal pole within a gestational sac.

Caution is advised to correctly differentiate a gestational sac from a pseudogestational sac, which represents fluid in the uterine cavity and is associated with an ectopic pregnancy.

Once an intrauterine gestation is confirmed, the focus shifts to fetal viability, in two ways. First, the size of the gestational sac is correlated with the fetal structures visualized. A mean sac diameter (MSD) is used to measure the gestational sac and is the average of 3 orthogonal measurements. Using a transvaginal approach, a fetal pole should be seen with a MSD of 25 mm. The second way to determine viability is to detect fetal heart motion within the fetal pole. Using a transvaginal approach for measurement, fetal heart motion should be seen with a crown rump length of 7 mm (see image below). These guidelines were updated at the Society of Radiologists in Ultrasound Multispecialty Consensus Conference on Early First Trimester Diagnosis of Miscarriage and Exclusion of a Viable Intrauterine Pregnancy in October 2012. [13]

A study found that current guidelines regarding ultrasonographic diagnosis of miscarriage may still be associated with misdiagnoses and should be updated to take into account gestational age. [14, 15]

Normal fetal crown rump measurement Normal fetal crown rump measurement

Keep in mind that any measurement taken with ultrasound has inherent errors; therefore, findings suggestive of a failed pregnancy should always be interpreted with caution and correlated with history, serial beta–human chorionic gonadotropin (hCG) values, and prior or follow-up imaging examinations.


Second- and Third-Trimester Ultrasound Evaluation

Second- and third-trimester ultrasound examinations are usually performed transabdominally with a curvilinear 3- to 5-MHz transducer. A lower-frequency transducer can be used in obese patients in order to obtain deeper beam penetration. However, resolution suffers with a lower-frequency beam. A translabial or transvaginal examination can be performed to better evaluate the cervix, if indicated.

With any second- or third-trimester ultrasound, the following items are addressed: [2]

  • Fetal number, presentation, and cardiac activity

  • Amniotic fluid assessment, either qualitatively or quantitatively (with maximal vertical pocket measurement or amniotic fluid index)

  • Placental location and relationship to internal cervical os

  • Cervical length

  • Gestational age measurement with comparison to clinical dating

  • Fetal weight estimation

  • Maternal uterus and adnexa

Multiple-gestation examinations also include evaluation of chorionicity, amnionicity, placental number, and fetal sex. The amniotic fluid and fetal growth of the fetuses are compared.

The following parameters are used to predict gestational age:

  • Biparietal diameter: Measured at the level of the cavum septum pellucidum (CSP) and thalami

  • Head circumference: Also measured at the level of the CSP and thalami (see image below)

    Head circumference and biparietal diameter measure Head circumference and biparietal diameter measurements, which are taken appropriately at the level of the thalamus and cavum septum pellucidum (arrow).
  • Femoral diaphysis length: Measured perpendicular to the ultrasound beam; the epiphyses are excluded (see image below)

    Femoral diaphysis measurement. Note that the measu Femoral diaphysis measurement. Note that the measurement is taken perpendicular to the axis of the ultrasound beam.
  • Abdominal circumference: Measured at the level of the umbilical vein, portal sinus, and stomach

The above measurements are then converted into an estimated fetal weight. Various prediction models are routinely available to obtain percentile rankings of estimated fetal weight based on gestational age. Errors in fetal weight estimation can be as high as ±15%, [2] likely because of ultrasound technical factors, as well as an inherent variability from the patient population and weight range being studied.


The Anatomic Survey

A fetal anatomic survey can be performed in the second trimester, usually between 18 and 22 weeks. Results are limited outside this range, as early imaging is obscured by lack of appropriate fetal organ development and delayed imaging is obscured by shadows created from bone ossification. The anatomic survey is performed in addition to the scan data described in Second- and Third-Trimester Ultrasound Evaluation.

The anatomic landmarks evaluated are discussed below.

  • Head: The lateral ventricles are measured at the level of the atria at the posterior choroid. A normal choroid plexus filling the ventricle in a transverse dimension is important in the evaluation for ventriculomegaly. The cavum septum pellucidum is scrutinized. In the posterior fossa, a normal-appearing cerebellum is noted. The cisterna magna and nuchal fold thickness are measured. (See Ultrasound Evaluation of the Fetal CNS.)

    A (top image): Normal nuchal fold and cisterna mag A (top image): Normal nuchal fold and cisterna magna measurements. Note other structures are also visualized: cerebellum (thin arrow), thalamus (thick arrow), and CSP (arrowhead). B (bottom image): Thickened nuchal fold, measuring 6 mm (normal < 6 mm before 22 weeks). This finding carries an increased risk of aneuploidy.
  • Face: Static images of the nasal bone and facial profile are taken.

    A (top image): Normal nasal bone (arrow). B (botto A (top image): Normal nasal bone (arrow). B (bottom image): Note the absence of a nasal bone (curved arrow) in a different patient.
  • Chest: A 4-chamber view of the heart is obtained, and the heart's position in the chest cavity is observed. Imaging of the outflow tracts is recommended but not required. The lungs should be homogenously echogenic, and evaluation for intrathoracic masses or cysts is performed. The diaphragm can be visualized to exclude any diaphragmatic hernia.

  • Abdomen: The stomach is identified and its position with reference to the heart is noted. The kidneys and urinary bladder are evaluated for obstruction. The umbilical cord is scrutinized, looking at its vessels and its abdominal insertion. The bowel is heterogeneous in appearance, but its echogenicity is important, as highly echogenic bowel (as bright as bone) is associated with aneuploidy. Dilated, fluid-filled bowel loops could indicate obstruction or atresia.

  • Spine: The spine and sacrum are examined for ossification centers, alignment, and paravertebral masses.

  • Extremities: The extremities are evaluated for abnormalities in size, morphology, and number.

  • Sex: Sex is determined as relevant in the settings of multiple gestations (to exclude monochorionicity) and renal pelviectasis (since posterior urethral valves are found exclusively in males).

Increased nuchal translucency at 11-14 weeks’ gestation and/or increased nuchal fold thickening during the anatomic survey is associated with an increased risk for aneuploidy. When markers for aneuploidy exist, they usually instigate further workup and/or correlation with laboratory data and risk factors. Although they are nonspecific, markers associated with certain chromosomal abnormalities are as follows: [16]

  • Turner syndrome (45, X): Cystic hygromas and coarctation of the aorta; the lethal type is also associated with pleural effusion, ascites, and cardiac defects

  • Trisomy 13 (Patau syndrome): Polydactyly, microcephaly, holoprosencephaly, cleft lip/palate, ocular anomalies, neural tube defects, and cardiac defects

  • Trisomy 18 (Edwards syndrome): Choroid plexus cyst, overlapping fingers, abnormal corpus callosum, strawberry-shaped head, micrognathia, omphalocele, diaphragmatic hernia, clenched hands, radial ray anomalies, clubfeet, and rocker bottom feet

  • Trisomy 21 (Down syndrome): Brachycephaly, cardiac defects (especially atrioventricular canal defects), echogenic bowel, short femur/humerus, renal pelviectasis, absent nasal bone, and sandal gap deformity of the feet

Follow-up examinations after the anatomic survey occur for a number of reasons. For instance, renal pelviectasis and placenta previa are usually monitored, as these conditions can regress or worsen over the course of the pregnancy. Suboptimal evaluation of any of the above described anatomic structures can also prompt follow-up, since fetal positioning can change, and therefore visualization can improve. Good communication between the radiologist, the ordering provider, and, at times, the maternal-fetal medicine department is essential to providing appropriate and adequate follow-up care for the pregnant patient.


Ultrasound Evaluation of the Fetal CNS

Guidelines by the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) describe sonographic screening evaluation of the fetal brain as part of the midtrimester anomaly screening examination and elaborate on the indications for detailed evaluation of the fetal central nervous system (CNS) by targeted fetal neurosonography, which has greater diagnostic potential but requires a high level of skill and expertise. [17, 18]

Those who are doing screening for CNS abnormalities should be familiar with the normal appearance of the CNS at different gestational ages. Transabdominal sonography is the technique of choice for screening the fetal CNS in low-risk pregnancies at the midtrimester scan and should include the fetal head and spine.

In the transventricular plane, the aspect of the atrium distal to the transducer and the presence of the cavum septi pellucidi should be evaluated. In the transcerebellar plane, the presence and shape of the cerebellum and the presence of the CSF in the cisterna magna should be evaluated. When possible, obtain a longitudinal section of the fetal spine to screen for open and closed spinal dysraphism.

Measure atrial width and transverse cerebellar diameter for CNS malformations. For general biometry purposes, biparietal diameter and head circumference are usually also measured because they may identify proliferation abnormalities such as microcephaly or macrocephaly.

Perform targeted fetal neurosonography if there is suspicion of a brain or spinal abnormality during obstetric ultrasound. Fetal brain MRI should be indicated only by the findings of the expert who performed the targeted neurosonographic exam, not on the basis of suspicion of a brain abnormality at screening ultrasound.

The transvaginal approach is preferred for high‐resolution targeted neurosonography. The transabdominal approach can be used when the transvaginal approach is not feasible because of situations such as breech presentation or twin pregnancy. When a transvaginal approach is not feasible, use high‐resolution linear or microconvex transducers because they provide higher resolution than conventional convex probes.

The midsagittal or median plane is the reference plane to assess major midline organs and anomalies and should be approached via the anterior or posterior fontanelle or the sagittal nonossified suture by gentle manipulation of the fetal head using the free hand.

Check that all 4 components of the corpus callosum are visible and sonographically normal.

3D ultrasound is recommended in targeted neurosonography. Fetal brain MRI is considered complementary to neurosonography. When necessary, it can replace neurosonography as the second‐line evaluation if the operator has sufficient training in fetal brain MRI.