The Role of Sonography in Obstetrics

Indications for Obstetric Sonography

The sonographer needs to be aware of the indications for obstetric sonography and to understand the medical complications associated with each indication. Recommended indications for obstetric sonography examinations are incorporated into diagnosis codes and billing codes. The AIUM, ACR, ACOG, SMFM, and Society of Radiologists in Ultrasound (SRU) first defined these indications in 2018. Current practice guidelines include first-trimester obstetric sonography and second- and third-trimester obstetric sonography. These indications are listed in Boxes 47.1 and 47.2 . An additional explanation for these indications is provided in the following paragraphs.

• 1.

  Estimation of gestational (menstrual) age for patients with uncertain clinical dates or verification of dates for patients who undergo scheduled elective repeat cesarean delivery, indicated induction of labor, or elective termination of pregnancy. Sonographic confirmation of dating permits proper timing of cesarean delivery or labor induction to avoid premature delivery.

• 2.

  Evaluation of fetal growth, for example, when the patient has an identified cause for uteroplacental insufficiency, such as severe preeclampsia, chronic hypertension, chronic renal disease, or severe diabetes mellitus, or for other medical complications of pregnancy in which fetal malnutrition (e.g., intrauterine growth restriction [IUGR] , or macrosomia ) is suspected. Measuring fetal growth by sonography at 2- to 4-week intervals permits assessing the impact of a complicating condition of the fetus and guides pregnancy management.

• 3.

  Vaginal bleeding of undetermined cause in pregnancy. Sonography often allows the determination of the source of bleeding and the status of the fetus.

• 4.

  Serial evaluation of cervical length in pregnant women with increased risk for recurrent preterm birth or primary preterm birth.

• 5.

  Evaluation of abdominal or pelvic pain in pregnancy that may be associated with ectopic pregnancy, abruptio placentae , or maternal appendicitis, gallstones, renal calculi, pelvic mass, or other conditions.

• 6.

  Determination of fetal presentation when the presenting part cannot be adequately determined in labor or the fetal presentation is variable in late pregnancy. Accurate knowledge of presentation guides management of delivery.

• 7.

  Suspected multiple gestation based on detection of more than one fetal heartbeat pattern, fundal height larger than expected for dates, or prior use of fertility drugs. Pregnancy management may be altered in multiple gestation.

• 8.

  Adjunct to amniocentesis , chorionic villous sampling (CVS), and other invasive pregnancy procedures. Sonography permits guidance to the intended target.

• 9.

  Significant discrepancy between uterine size and clinical dates. Sonography permits accurate dating and detection of such conditions as oligohydramnios and polyhydramnios , along with multiple gestation, IUGR, and anomalies.

• 10.

  Evaluation of pelvic mass. Sonography can detect the location and nature of the mass and can aid in diagnosis.

• 11.

  Hydatidiform mole suspected based on clinical signs of hypertension, proteinuria, or the presence of ovarian cysts felt on pelvic examination or failure to detect fetal heart tones with a Doppler ultrasound device after 12 weeks. Sonography permits accurate diagnosis and differentiation of this neoplasm from fetal death.

• 12.

  Adjunct to cervical cerclage placement. Sonography aids in timing and proper cerclage placement for patients with incompetent cervix .

• 13.

  Suspected ectopic pregnancy, or pregnancy that occurs after tuboplasty or prior ectopic gestation. Sonography is a valuable diagnostic aid for this complication.

• 14.

  Evaluation of suspected fetal death. Rapid diagnosis enhances optimal management.

• 15.

  Suspected uterine abnormality (e.g., clinically significant leiomyomas; congenital structural abnormalities, such as bicornuate uterus or uteri didelphys). Serial surveillance of fetal growth and state enhances fetal outcome.

• 16.

  Evaluation of fetal well-being. Biophysical evaluation for fetal well-being after 28 weeks of gestation may include assessment of amniotic fluid, fetal tone, body movements, breathing movements, and heart rate patterns.

• 17.

  Evaluation of suspected amniotic fluid abnormalities such as suspected polyhydramnios or oligohydramnios. Confirmation of the diagnosis and identification of the condition's cause in certain pregnancies are necessary.

• 18.

  Suspected abruptio placentae. Confirmation of diagnosis and extent of abruption assists in clinical management.

• 19.

  Adjunct to external version from breech to vertex presentation. The visualization provided by sonography facilitates performance of this procedure.

• 20.

  Estimation of fetal weight and presentation in premature rupture of the membranes or premature labor. Information provided by sonography guides management decisions on timing and method of delivery.

• 21.

  Evaluation following maternal serum biochemical marker results. Elevated maternal serum alpha-fetoprotein (MSAFP) increases the risk for open defects such as neural tube defects. Other biochemical markers in the first trimester or quad screen biochemistry in the second trimester may indicate increased risk for certain obstetric or fetal conditions.

• 22.

  Follow-up observation of identified fetal anomaly. Sonographic assessment of progression or lack of change may assist in clinical management.

• 23.

  Follow-up evaluation of placenta location for suspected placenta previa .

• 24.

  Evaluation for those with a history of previous congenital anomaly. Detection of recurrence may be facilitated, or psychological benefit to patients may result from reassurance of no recurrence.

• 25.

  Evaluation of fetal condition in late registrants for prenatal care. Assessment of gestational age and fetal size assists in pregnancy management decisions for this group.

• 26.

  Assessment of findings that may increase the risk of aneuploidy .

• 27.

  Screening for fetal anomalies by measuring the nuchal translucency or visualization of fetal structural anomalies.

Classification of Fetal Ultrasound Examinations

The four practice guidelines define the major types of sonographic examinations performed in the first trimester, second and third trimesters of pregnancy, using the terms limited, standard , and specialized or detailed . In practice, the examinations may also be referred to by the current procedure terminology (CPT) code most commonly used for billing of the examinations. The major types of obstetric sonography examinations are listed in Box 47.3 and are described in the following sections.

The first-trimester examination (CPT code 76801) is performed before 13 weeks and 6 days of gestation. The examination includes the uterus, the cervix , the maternal adnexa, and the gestational sac and embryo. The pregnancy is dated based on embryonic size, and fetal heart motion is documented if these findings are present. Uterine anomalies and pelvic masses associated with pregnancy are more easily seen in first-trimester examinations. The chorionicity and amnionicity of multiple gestations should be documented at this time as well. An examination of fetal anatomy should also be performed during first-trimester examinations.

The first-trimester risk assessment examination (CPT code 76813) is known as the nuchal translucency (NT) examination. This examination is performed only when women choose first-trimester screening tests for aneuploidy. The examination includes measurement of fetal crown-rump length and NT using standard criteria. In some centers, the examination may also include visualization of the fetal nasal bone and other risk assessment parameters. Sonographers who perform these examinations must demonstrate competence in the standardized measurement of NT and must participate in an ongoing quality-monitoring program.

The standard obstetric sonography examination (CPT code 76805) is typically performed during the second trimester, around 18 weeks of gestational age. The standard examination includes an evaluation of gestational age by fetal biometry, fetal number, fetal presentation, placental position, cardiac activity, amniotic fluid volume, and a fetal anatomic survey, including all of the elements specified in the guidelines. The standard examination may include the maternal cervix and adnexa when clinically appropriate. If the cervix cannot be visualized, a transvaginal scan may be considered when evaluation or measurement of the cervix is needed.

The limited obstetric sonography examination (CPT code 76815) is used when the answer to a specific acute clinical question when an immediate impact on management is anticipated and when time or other constraints make performance of a standard ultrasound examination impractical or unnecessary. A specific clinical question such as presentation of the fetus, placental location, cervical length, amniotic fluid volume, or verification of fetal heart motion is required. If a limited examination is done on a woman without a previous standard obstetrical ultrasound, a subsequent standard or detailed ultrasound exam should be performed when appropriate.

A repeat obstetric sonography examination (CPT code 76816) is similar to a standard obstetric examination and typically includes biometry to evaluate fetal growth and reevaluation of anatomy that may or may not have been well visualized on the standard examination. The repeat obstetric examination is done when a previous standard obstetric examination has been recorded and the second examination is ordered for the same indication.

The specialty obstetric sonography examination (CPT code 76811) is also known as a detailed fetal anatomic sonogram. It is performed when an anomaly is suspected based on maternal history, biochemistry, or the results of a previous obstetric sonogram, when there is a known fetal growth disorder, or when there is increased risk for a fetal condition. The specialty obstetric sonography examination includes all components of the standard examination plus a more in-depth view of fetal anatomy. The specialty examination typically includes additional views of the fetal heart and may include color Doppler views of the heart. The specialty examination may include additional views of the extremities and a focus on areas of anomalous or expected findings associated with the patient history. The specialty obstetric sonography examination is typically performed in referral centers by physicians and sonographers with specific expertise in high-risk obstetrics. A consensus report on the detailed fetal anatomic examination was developed in 2018 by representatives from AIUM, ACOG, ACR, the SMFM, the American College of Osteopathic Obstetricians and Gynecologists (ACOOG), the SRU, and the Society of Diagnostic Medical Sonography (SDMS).

Additional CPT codes are used for transvaginal obstetric examinations, multiple gestations, fetal echocardiography, three-dimensional (3D)/four-dimensional (4D) examinations, biophysical profiles, and invasive procedures. Sonographers performing obstetric sonography must know the components required for each type of examination. Health care compliance regulations require that the billing or CPT code must match the examination performed.

Patient History

The sonographer should ask the patient several important questions before beginning the obstetric sonography evaluation. Both open-ended questions such as “Do you have concerns?” and closed questions such as “When was your last normal period?” are used in gathering important patient information.