On completion of this chapter, you should be able to:
Discuss indications for obstetric sonography
Analyze the differences among standard, specialized, and limited obstetric sonography examinations
List maternal risk factors that increase the chances of producing a fetus with congenital anomalies
Recount important questions to ask the patient before beginning the obstetric sonography examination
Describe the biologic effects of diagnostic medical ultrasound energy and related patient safety
Describe the steps of the first-, second-, and third-trimester sonography protocols
List fetal anatomy visualization required as part of the standard second-trimester examination
Discuss the use of sonography as a diagnostic and screening test
Sonography is the primary tool for evaluating the developing fetus during pregnancy. Obstetric sonography allows the clinician to assess the development, growth, and well-being of the fetus. When an abnormal condition is recognized prenatally, obstetric management may be altered to provide optimal care for the fetus and mother. The visualization of pregnancy with sonography has revolutionized obstetrics. Conditions that were once detected only at delivery are now diagnosed early in pregnancy and monitored with sonography. Prenatal diagnosis has led to prenatal treatments performed under ultrasound visualization. Sharing of sonographic images and diagnostic results facilitates prenatal parental education and counseling. Although obstetric sonography is popular in many aspects of our culture, including books, television, and family gatherings, its value lies in its medical use.
The sonographer performing fetal studies must understand both sonographic and obstetric principles to accurately and thoroughly compile pertinent information and to provide an optimal sonographic assessment of the fetus. Fetal sonography should be performed only when there is a valid medical reason and using the lowest possible ultrasound energy exposure settings to gain the necessary diagnostic information. The sonographer has a responsibility to obstetric patients and clinicians to provide competent, safe, and appropriate examinations.
Practice parameters are guidelines produced by the American College of Radiology (ACR), the American Institute of Ultrasound in Medicine (AIUM), and the American College of Obstetricians and Gynecologists (ACOG) that recommend specific components of a standard obstetric sonography examination. Sonographers must strive during each examination to meet the recommended requirements. In addition, components may be altered or added to serve the interests of the patient or the referring clinician. It is often the responsibility of the sonographer, under the general direction of a physician, to apply knowledge, competence, and critical thinking to determine and perform additional appropriate examination components based on the specific indication for the study and the clinical history of the mother.
In accordance with recommendations, the sonographer should establish a systematic scanning protocol that encompasses all criteria indicated in the guidelines. An organized approach to scanning ensures completeness and reduces the risk of missing a detectable obstetric or fetal concern.
This chapter describes the medical indications for obstetric sonography examinations and the types of obstetric examinations performed; summarizes practice parameter guidelines; reviews the safety of ultrasound in obstetrics; and describes maternal risk factors and history that may alter examination protocols.
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 National Institute of Child Health and Human Development in the National Institutes of Health Consensus Report on Safety of Ultrasound first defined these indications in 1984. Current practice guidelines include indications for first-trimester obstetric sonography and second- and third-trimester obstetric sonography that were adapted from the 1984 list. These indications are listed in Boxes 47-1 and 47-2 . Additional explanation for these indications is provided in the following paragraphs.
Estimation of gestational (menstrual) age for patients with uncertain clinical dates or verification of dates for patients who are to 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.
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 assessment of the impact of a complicating condition of the fetus and guides pregnancy management.
Vaginal bleeding of undetermined cause in pregnancy. Sonography often allows determination of the source of bleeding and the status of the fetus.
Serial evaluation of cervical length in pregnant women with increased risk for recurrent preterm birth or primary preterm birth.
Evaluation of abdominal or pelvic pain in pregnancy that may be associated with ectopic pregnancy, abruptio placentae, or maternal appendicitis, renal calculi, pelvic mass, or other conditions.
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.
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.
Adjunct to amniocentesis, chorionic villi sampling (CVS) and other invasive pregnancy procedures. Sonography permits guidance to the intended target.
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.
Evaluation of pelvic mass. Sonography can detect the location and nature of the mass and can aid in diagnosis.
Hydatidiform mole suspected on the basis of 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.
Adjunct to cervical cerclage placement. Sonography aids in timing and proper placement of the cerclage for patients with incompetent cervix.
Suspected ectopic pregnancy, or pregnancy that occurs after tuboplasty or prior ectopic gestation. Sonography is a valuable diagnostic aid for this complication.
Evaluation of suspected fetal death. Rapid diagnosis enhances optimal management.
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.
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.
Evaluation of suspected amniotic fluid abnormalities such as suspected polyhydramnios or oligohydramnios. Confirmation of the diagnosis and identification of the cause of the condition in certain pregnancies are necessary.
Suspected abruptio placentae. Confirmation of diagnosis and extent of abruption assists in clinical management.
Adjunct to external version from breech to vertex presentation. The visualization provided by sonography facilitates performance of this procedure.
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.
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.
Follow-up observation of identified fetal anomaly. Sonographic assessment of progression or lack of change may assist in clinical management.
Follow-up evaluation of placenta location for suspected placenta previa.
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.
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.
Assessment of findings that may increase the risk of aneuploidy.
Screening for fetal anomalies through measurement of the nuchal translucency or visualization of fetal structural anomalies.
To confirm the presence of an intrauterine pregnancy
To evaluate a suspected ectopic pregnancy
To define the cause of vaginal bleeding
To evaluate pelvic pain
To estimate gestational (menstrual) age
To diagnose or evaluate multiple pregnancy
To confirm cardiac activity
As an adjunct to chorionic villous sampling, embryo transfer, or localization and removal of an intrauterine device
To assess for certain fetal anomalies, such as anencephaly, in patients at high risk
To evaluate maternal pelvic or adnexal masses or uterine abnormalities
To screen for fetal aneuploidy by measuring the nuchal translucency
To evaluate suspected hydatidiform mole
Types of obstetric sonography examinations
The three practice guidelines define the major types of sonographic examinations performed in the 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.
First-Trimester Examination (CPT 76801)
First-Trimester Nuchal Translucency (CPT 76813)
Standard Obstetric Examination (CPT 76805)
Repeat Obstetric Examination (CPT 76816)
Limited Obstetric Examination (CPT 76815)
Detailed Obstetric Examination (CPT 76811)
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 as well 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 clinical question such as presentation of the fetus, placental location, cervical length, amniotic fluid volume, or verification of fetal heart motion is required. A limited examination is done when a previous standard obstetric examination has been recorded.
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 2013 by representatives from AIUM, ACOG, ACR, the Society of Maternal Fetal Medicine (SMFM), the American College of Osteopathic Obstetricians and Gynecologist (ACOOG), the Society of Radiologist in Ultrasound (SRU), and the Society of Diagnostic Medical Sonography (SDMS).
The first-trimester examination (CPT code 76801) is performed before 13 weeks and 6 days of gestation. The examination includes the uterus, the cervix, and the maternal adnexa, as well as 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 also known as the nuchal translucency 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 measurement of nuchal translucency 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 nuchal translucency and must participate in an ongoing quality-monitoring program.
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.
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.
Gravidity and parity
Key obstetric history of the patient is summarized using gravidity (G) and parity (P). The sonographer should recognize this clinical description of the pregnant patient. Gravidity is the number of pregnancies, including the present one. Parity is reported using a numeric system that describes all possible pregnancy outcomes. The letter “P” followed by four numbers in sequence, P0000, is commonly used. The numbers represent, in order, full-term deliveries, premature births and stillborns, early pregnancy loss or termination, and living children. For instance, a G4P2103 describes a patient undergoing her fourth pregnancy. She has had two full-term deliveries, one premature birth, no early pregnancy losses, and three living children.
The sonographer first tries to determine the clinical dates of the pregnancy. It is important to document the clinical date reported by the patient and the date determined by the earliest sonographic examination. An accurate clinical date facilitates correlation of obstetric measurements with the expected gestational age.
The first date of the last normal menstrual period (LMP or LNMP) is the standard way to date a pregnancy in the United States. Human pregnancy lasts 266 days plus or minus 10 days. If conception occurs on day 14 from the LNMP, the pregnancy duration from LNMP is 280 days or 40 weeks. Pregnancy is divided into trimesters of approximately 13 weeks. A pregnant woman is in the first trimester until 13 weeks and 6 days of gestational age, and in the second trimester from 14 weeks to 26 weeks and 6 days of gestational age. The third trimester begins at 27 weeks of gestational age and lasts until term.
In reality, the assessment of gestational age is often not precise. Many women have irregular periods, conceive within 3 months of coming off birth control pills when ovulation is irregular, or do not record dates. Even with a known menstrual date, conception may occur from day 6 to day 27, which is a difference of 3 weeks in gestational age as determined by sonography. Physicians may use clinical parameters such as uterine size and growth or ovulation indications to estimate pregnancy dates. Gestational age provides an estimate of how long a patient has been pregnant, but the exact date that labor will begin cannot be determined owing to the variable length of human pregnancy.
The sonographer first asks the patient the first day of her last menstrual period. If the patient does not remember the date of her LNMP, the sonographer may ask for the expected date of delivery (EDD). The sonographer should also ask if previous sonographic examinations were performed before 20 weeks and the estimated date of delivery determined by the earliest sonographic examination. The sonographer should not change the EDD or “redate” the pregnancy once an earlier sonogram has established gestational age.
Dates established by sonography performed in the first or second trimester typically take precedence over menstrual dates when the discrepancy is greater than 7 days in the first trimester or greater than 10 days in the second trimester. Sonography may be considered to confirm menstrual dates if there is gestational age agreement within a week by crown-rump length or within 10 days by second-trimester fetal biometry. The pregnancy should not be dated by sonographic measurements in the third trimester, and dates should not be changed after they have been calculated from an early examination. It is ultimately the responsibility of the obstetrician who is following the pregnancy to determine the clinical gestational age.
The EDD may be calculated using Nägele’s rule ( Box 47-4 ). According to this method, the EDD is derived by subtracting 3 months from the LNMP and adding 7 days. For example, an LNMP of 10/17 would result in an EDC of 7/24 (10/17 − 3 months = 7/17 + 7 days = 7/24). A sonographer familiar with this rule may determine EDD or LNMP when the patient verbally reports only one. Commercial date wheels simplify this method to determine the due date and to assign fetal age at the time of the sonography study.