Indications for diagnostic obstetrical ultrasound examination
Jeffrey L Ecker, MD
Michael F Greene, MD
UpToDate performs a continuous review of over 375 journals and other resources. Updates are added as important new information is published. The literature review for version 15.1 is current through December 2006; this topic was last changed on December 29, 2006. The next version of UpToDate (15.2) will be released in June 2007.
INTRODUCTION — Ultrasound imaging has dramatically changed the practice of obstetrics by enabling visualization of the fetus and fetal environment. In the United States and many other countries, ultrasound is a routine component of prenatal care.
The degree to which ultrasound improves pregnancy care and outcome is controversial and depends upon whether the procedure is used as a diagnostic or screening test. As a diagnostic procedure, ultrasound is used to address defined clinical questions and helps to make or exclude diagnoses suggested clinically (eg, placenta previa in a gravida with vaginal bleeding). As a screening test, ultrasound is employed in low-risk patients to identify pregnancies that might benefit from further testing or intervention (eg, karyotype or termination for an unsuspected fetal anomaly). The use of sonography as a screening tool is reviewed separately. (See "Routine prenatal ultrasonography as a screening tool").
An overview of diagnostic obstetrical ultrasound examination will be presented here. Specific topics are discussed in depth within individual topic reviews on each subject.
DIAGNOSTIC ULTRASOUND — A consensus development workshop convened by the National Institute of Child Health and Human Development in 1984 concluded there were over two dozen indications for ultrasonography during pregnancy [1]. This list is shown in the table, with the addition of a few updated indications from the American College of Radiology (show table 1) [1,2].
Nonmedical use of obstetric ultrasonography, such as to provide "baby pictures" or videos for parents or to determine fetal sex when there is no medical indication for this knowledge, has been discouraged by both the American College of Obstetricians and Gynecologists (ACOG) and the American Institute of Ultrasound in Medicine (AIUM) [3]. These organizations are concerned that nonmedical ultrasound examinations can be misinterpreted by patients and could be falsely concerning or falsely reassuring.
Components of a basic examination
  First trimester — The goals of the first trimester examination are to [4]: Confirm the presence of an intrauterine (or evaluate suspected extrauterine) pregnancy Determine if fetal cardiac activity is present Assess gestational age Determine whether a multiple gestation is present, and its chorionicity/amnionicity Evaluate maternal pelvic organs for congenital or acquired abnormalities.
  Second or third trimester — A basic second or third trimester ultrasound examination, which will suffice for most patients, provides the following information [4,5]: Fetal number Fetal presentation Documentation of fetal cardiac activity, including heart rate Placental location Assessment of amniotic fluid volume Fetal biometry for assessment of gestational age and weight Survey of fetal anatomy (show table 2) Evaluation of maternal pelvic organs
There are several methods of amniotic fluid volume assessment, which are discussed elsewhere. (See "Assessment of amniotic fluid volume").
In addition, the American College of Radiology has recommended that the cervix and lower uterine segment be imaged in every second trimester examination to look for funneling or a short cervix [6], although the clinical utility of such evaluation in a general population remains controversial. (See "Prediction of prematurity by transvaginal ultrasound assessment of the cervix").
The AIUM also recommends visualization of the umbilical cord, its insertion site in the fetal abdomen, and evaluation of the number of cord vessels, when possible [4].
When fetal abnormalities are detected, a more detailed or comprehensive examination by a sonologist with training in evaluating such pregnancies is indicated.
Limited ultrasound examinations can be performed to address specific questions in patients who have been previously evaluated by a complete examination. Examples of appropriate use of limited studies include confirmation of the presence or absence of fetal cardiac activity, checking fetal presentation, and assessment of amniotic fluid volume in conjunction with nonstress testing.
Procedure — (See "Ultrasound examination in obstetrics and gynecology: Procedure").
Safety — (See "Physics and safety of diagnostic ultrasound in obstetrics and gynecology").
FIRST TRIMESTER BLEEDING — Vaginal bleeding complicates approximately 20 to 40 percent of early pregnancies. The major differential diagnoses of bleeding during this period of pregnancy are related to ectopic pregnancy and spontaneous abortion. Gestational trophoblastic disease is also a consideration. (See "Overview of the etiology and evaluation of vaginal bleeding in pregnant women").
Suspected ectopic pregnancy — Ectopic implantation, most commonly in the fallopian tube, occurs in 1.6 percent of all pregnancies and is a significant cause of maternal morbidity and mortality. An untreated tubal pregnancy can result in tubal rupture with potentially life threatening hemorrhage; thus, clinicians should always have a high index of suspicion for this entity whenever a woman presents with bleeding and/or abdominal pain early in gestation. (See "Incidence, risk factors, and pathology of ectopic pregnancy").
A woman rarely has both an intrauterine and concomitant extrauterine gestation (ie, heterotopic pregnancy 1/30,000 spontaneous conceptions), therefore the identification of an intrauterine pregnancy effectively excludes the possibility of an ectopic in almost all cases. However, pregnancies conceived with assisted reproductive technology are an exception, since the incidence of heterotopic pregnancy may be as high as 1/100 to 1/3000 pregnancies [7]. (See "Incidence, risk factors, and pathology of ectopic pregnancy", section on Heterotopic pregnancy).
The combined use of transvaginal sonography of the uterus/adnexa and serum hCG levels allows a definitive diagnosis in the vast majority of cases at a very early stage, when there is a high likelihood of successful conservative (medical) treatment.
The clinical manifestations and diagnosis of ectopic pregnancy are discussed in detail separately. (See "Clinical manifestations, diagnosis, and management of ectopic pregnancy", section on Diagnostic evaluation).
Suspected spontaneous abortion — Bleeding early in pregnancy may also be due to a threatened, missed, incomplete, or complete spontaneous abortion. Fifteen to 30 percent of recognized pregnancies end in early pregnancy loss; ultrasound can help in diagnosis of such losses. (See "Spontaneous abortion: Risk factors, etiology, clinical manifestations, and diagnostic evaluation", section on Ultrasonography).
ESTIMATION OF GESTATIONAL AGE — Ultrasound is used to estimate gestational age and thereby calculate the expected date of delivery (EDD). Sonographic estimation may be particularly important when menses are irregular, the LMP is unknown, or in patients conceiving while taking oral contraceptive pills. Ultrasound may also establish a pregnancy's duration when the uterine size estimated on physical examination differs from that predicted by menstrual dating. Some causes for a discrepancy between the actual uterine size and that expected by LMP include uterine fibroids, uterine malposition (eg, retroverted uterus), and multiple gestation. Use of ultrasound examination to assess gestational age is discussed in detail separately. (See "Prenatal assessment of gestational age and fetal weight").
MULTIPLE GESTATION AND ZYGOSITY — Ultrasound is indicated to establish the number and zygosity of fetuses when multiple gestation is suspected. Assisted reproductive technology, family history of twins (or higher multiples), and uterine size larger than that expected by menstrual dating are risk factors for multiple gestation. The number of fetuses can be clearly established by visualizing the number of fetal poles with distinct cardiac activity. An image that includes a cross-section of all fetal poles within a single frame should be obtained to reduce the possibility that a fetus will be counted more than once if it is imaged from a different angle.
Establishing the zygosity of such pregnancies is more difficult, but should be performed because monochorionic multiple gestations pose special risks that may require expert management during pregnancy. Sonographic signs that can be used to help determine amnionicity and chorionicity include counting the number of fetal yolk sacs, determining fetal sex, assessing placental position and separation, and evaluating the presence and appearance of any inter-twin membranes [8-11]. (See "Antepartum assessment of twin gestations", section on Amnionicity and chorionicity).
CONGENITAL ANOMALIES — Fetal anatomy is generally best evaluated by ultrasound examination between 16 and 24 weeks of gestation. Some anomalies are best visualized near the end of this range while others can be identified even earlier; it may be possible to document normal or abnormal structures in the first trimester, but fetal size and development limit optimal visualization. Transvaginal imaging may be useful if early visualization is needed either because a patient is at increased risk for an anomaly or because transabdominal images suggest an abnormality. Later in pregnancy, fetal position and movement, maternal abdominal scars or obesity, and abnormalities in amniotic fluid volume may pose technical limitations to anatomic evaluation.
Structural abnormalities that can be reliably diagnosed by an ultrasound examination include hydrocephalus, anencephaly, myelomeningocoele, dwarfing syndromes, spina bifida, omphalocele, gastroschisis, duodenal atresia and fetal hydrops, cleft lip, clubfoot, many urinary tract and renal abnormalities, and a variety of congenital cardiac abnormalities (see individual topic reviews).
First trimester ultrasonic markers for chromosomal abnormalities such as increased fetal nuchal translucency are used together with serum analytes to help with detection of Down syndrome fetuses. (See "The sonographic diagnosis of fetal aneuploidy" and see "First trimester screening for Down syndrome and trisomy 18").
Ultrasound may be able to identify congenital abnormalities in pregnancies in which either exposure to teratogens, a family's medical history, or prenatal screening tests raise specific concerns for an anomaly. As an example, the risk of recurrence for Meckel Gruber Syndrome is as high as 25 percent; the diagnosis should be suspected prenatally if there is sonographic visualization of occipital cephalocele, bilateral polycystic kidneys, and/or post-axial polydactyly.
The performance characteristics of ultrasound for detecting congenital abnormalities varies depending upon the specific anatomic defect, the gestational age at the time of the procedure, the skill of the ultrasonographer, and whether the population is at high or low risk for anomalies [12-15].
The routine use of ultrasound for screening low risk populations for fetal anomalies is discussed separately. (See "Routine prenatal ultrasonography as a screening tool", section on Detection of congenital anomalies).
SECOND AND THIRD TRIMESTER BLEEDING — The major placental causes of second and third trimester bleeding are placenta previa and abruptio placentae. Other obstetrical causes of bleeding in late pregnancy include cervical change associated with cervical insufficiency, rupture of membranes, or labor and vasa previa.
Placental localization — An obstetric ultrasound examination to identify placental location is useful in pregnancies in which there is second or third trimester bleeding (show ultrasound 1). Transvaginal technique to visualize the cervix and placental edge can be performed safely even when placenta previa is present and may provide detailed images important in making a diagnosis of placenta previa [16]. (See "Placenta previa and vasa previa", section on Ultrasonography).
Abruptio placentae — Placental separation (ie, abruptio placenta) is another cause of bleeding in late pregnancy. It may be associated with uterine pain, painful contractions, coagulopathy, abdominal trauma, hypertension, or use of cocaine. The diagnosis of placental abruption is primarily based upon the presence of characteristic clinical manifestations and the absence of a placenta previa on ultrasound examination. Sonographic evaluation will detect placental separation in only 30 percent of abruptions that are either suspected clinically or later recognized on pathologic examination of the placenta [17]. Therefore, a normal ultrasound examination should not change a clinical diagnosis of abruption. (See "Clinical features and diagnosis of abruptio placentae", section on Sonography).
Placenta accreta — Ultrasound may be of value for predicting the presence of placenta accreta, increta, or percreta [18]. Uterine scarring resulting from prior surgery such as cesarean section, myomectomy, or treatment of Asherman syndrome are risk factors for abnormal placentation. Visualization of a clear border between the placenta and the underlying myometrium suggests that the placenta has not invaded to or beyond the myometrium. In contrast, an indistinct border between the chorion and the myometrium is suggestive of a placenta that has grown through the myometrial wall into surrounding visceral structures. Occasionally, placental tissue can be seen growing directly into the adjacent bladder. This information is important in planning an approach to operative delivery. (See "Diagnosis and management of placenta accreta", section on Ultrasonography).
ASSESSMENT OF FETAL WEIGHT — Investigators have developed several formulas using a variety of sonographically obtained biometric measurements (eg, head circumference, femur length, abdominal circumference, cerebellar diameter, subcutaneous adipose tissue) to estimate fetal weight in the late second and the third trimester. In general, most formulas focus on measurement of biparietal diameter and abdominal circumference. Fetal weight estimates obtained from these equations are then compared to distributions normalized for gestational age and, ideally, a patient's individual ethnic background. However, all such formulas have a large range of error in estimation of fetal weight.
Techniques for sonographic estimation of fetal weight are discussed in detail separately. (See "Prenatal assessment of gestational age and fetal weight").
Intrauterine growth restriction — Fetal growth limiting disorders are more likely when the uterine size is less than appropriate for gestational age, a prior pregnancy has been affected by growth restriction, or in the presence of maternal risk factors such as hypertension. Ultrasound is employed to diagnose fetal growth restriction and to assess growth in pregnancies with multiple gestations, as these fetuses are at risk for growth restriction late in pregnancy and are difficult to assess by physical examination.
A sonographic finding of a physically small fetus does not distinguish those fetuses that are constitutionally small from those that are pathologically so (eg, from uteroplacental insufficiency). It is useful to know the etiology of the growth disturbance since some problems are progressive and likely to benefit from close monitoring and possible early intervention. Decreased amniotic fluid volume, a lower than expected amount of interval growth, and evidence of abnormal blood flow through the umbilical cord (Doppler flow studies) can increase the positive predictive value of a diagnosis of utero-placental insufficiency [19].
Clinicians should be circumspect about making the diagnosis of growth restriction when gestational age is uncertain. While it may be tempting in such cases to conclude that the menstrual dating is in error, particularly if other testing such as amniotic fluid volume and umbilical artery Doppler studies are normal, practitioners should treat such fetuses as possibly growth restricted and obtain fetal surveillance and follow-up ultrasound examinations, as indicated.
The sonographic diagnosis of fetal growth delay is discussed in detail separately. (See "Fetal growth restriction: Diagnosis").
Macrosomia — Ultrasound examination can also be unreliable in estimating fetal weight at the upper percentile for gestational age. The value of obtaining ultrasound examination for suspected macrosomia has been widely debated in the obstetric literature [20]. Decision analysis examining the utility of ultrasound for the prediction of macrosomia in the general obstetric population argues that it is neither clinically useful (sensitivity 60 percent, specificity 90 percent), nor cost effective [21]. (See "Fetal macrosomia: Diagnosis").
However, there may be some patients in whom the risk for birth trauma due to macrosomia is so great that intervention based upon the sonographic diagnosis of macrosomia may be appropriate. Many practitioners, as an example, offer prophylactic cesarean delivery to women with diabetes in whom sonographic estimation of fetal weight is greater than 4500 grams or women without diabetes in whom the estimated weight is greater than 5000 grams [22]. (See "Diagnosis and management of pregnancies at risk for shoulder dystocia").
ASSESSMENT OF FETAL WELL-BEING — The goals of antepartum fetal assessment are to avoid unnecessary interventions (especially preterm delivery) when the fetus appears to be healthy and to facilitate timely intervention when the fetal status is not reassuring, and thereby reduce morbidity/mortality.
There are a number of situations in which ultrasound is used alone or in combination with other tests (eg, nonstress test) to assess fetal well-being. Some examples of these situations include: Fetal conditions such as congenital anomalies, suspected infection, or hydrops fetalis Intrauterine growth restriction Maternal medical conditions such as diabetes mellitus, chronic hypertension, renal disease, hemoglobinopathy, collagen vascular disease, and antiphospholipid syndrome Oligohydramnios Complications of pregnancy such as bleeding or preeclampsia Decreased fetal movement Postterm pregnancy Isoimmunization
The urgency and frequency of testing depends upon the underlying condition. The advantage of sonography over antepartum fetal heart rate testing is that it provides information about fetal growth and amniotic fluid volume, parameters of particular importance when uteroplacental insufficiency is suspected.
Biophysical profile — The biophysical profile (BPP) refers to the sonographic assessment of four fetal parameters: fetal breathing motion, fetal activity, fetal muscular tone, and amniotic fluid volume (show table 3). Each parameter is scored by the sonographer with either zero or two points. A reactive nonstress test is often incorporated as an additional two point measurement, making a score of 10 the maximum possible points in the panel (normal score is 8 to 10). This series of measurements was selected, in part, because they are easily obtained by ultrasound examination and they mirror subsequent postnatal evaluation of neonatal well-being, such as Apgar scores. (See "The fetal biophysical profile").
Modified biophysical profile — The modified BPP was developed to simplify the examination and reduce the time necessary to complete testing by focusing on those components of the profile that are most predictive of outcome. Assessment of amniotic fluid volume and nonstress testing appear to be as reliable a predictor of longterm fetal well-being as the full BPP [23]. A normal modified biophysical profile will occur in 90 percent of pregnancies tested, thus it is necessary to proceed with a full biophysical evaluation in only a minority of patients [24]. (See "The fetal biophysical profile", section on Modified biophysical profile).
Doppler velocimetry — Fetal well-being may also be assessed by measuring umbilical artery blood flow via Doppler velocimetry. The underlying principle for such measurements is that fetuses with placental vasculopathy will demonstrate decreased umbilical artery flow during fetal diastole due to increased resistance (or high impedance) in fetal villus vessels. Umbilical artery flow is described quantitatively as the quotient of systolic/diastolic flow or by using other similar ratios. (See "Doppler ultrasound of the umbilical artery for fetal surveillance").
Doppler flow studies are especially useful in pregnancies complicated by growth restriction. (See "Fetal growth restriction: Evaluation and management").
Doppler measurement of flow in the fetal middle cerebral artery is a reliable substitute for more invasive measurements of fetal anemia [25]. (See "Diagnosis and management of Rhesus (Rh) alloimmunization", section on Doppler velocimetry).
Evaluation of cardiovascular status by arterial Doppler alone is inadequate in fetal disorders with impaired cardiac function since cardiac function is not accounted for in arterial waveform analysis. Extending Doppler ultrasound assessment to the fetal venous circulation overcomes this limitation. (See "Venous Doppler for fetal assessment").
HYDROPS FETALIS — Hydrops fetalis refers to the accumulation of fluid in fetal tissues or body cavities (eg, skin edema, ascites, pleural or pericardial effusions). It is the end result of a number of immune and nonimmune mediated fetal pathologic conditions. Serial ultrasound examinations appear to be useful for following pregnancies at risk for developing hydrops or to evaluate the effects of treatment on the course of the disorder. (See "Nonimmune hydrops fetalis").
DETERMINATION OF FETAL PRESENTATION AND POSITION — Fetal presentation, the part of the fetus overlying the maternal cervix, is easily determined by ultrasound examination. Practitioners may suspect a noncephalic presentation based upon physical examination (Leopold maneuvers) (show figure 1), a history of uterine anomaly, or palpation of the fetal breech or extremity at the time of cervical examination. Recognition of a breech presentation prior to labor is valuable, in part, because external cephalic version decreases the number of noncephalic presentations at term and the number of cesarean deliveries performed for this indication [26]. (See "Approach to breech presentation").
The position of the fetus (eg, occiput posterior) can also be determined sonographically. Studies have consistently shown that ultrasound examination of fetal position is more accurate than vaginal or abdominal examination [27-30]. As an example, one study assessed fetal presentation clinically and by ultrasound in 1633 nonlaboring women with singleton pregnancies at 35 to 37 weeks of gestation [31]. Ultrasonography identified noncephalic presentation in 130 women (8 percent); the sensitivity of clinical examination for detecting noncephalic presentation was 70 percent (95%CI 62-78) and specificity was 95 percent (95%CI 94-96). (See "Abnormal labor: Protraction and arrest disorders", section on Occiput posterior position).
EXAMINATION OF THE CERVIX — The cervix may be imaged and its length measured using ultrasound. Transvaginal technique is recommended to improve accuracy and reproducibility of such measurements [32]. The appearance of the amniotic membranes in relation to the internal cervical os and endocervical canal may also be evaluated. Membranes that protrude through an opened internal os are described as funneling. Both a short cervix and the presence of funneling on second and early third trimester ultrasound examination are associated with an increased risk of preterm delivery. (See "Prediction of prematurity by transvaginal ultrasound assessment of the cervix").
Routine ultrasound measurement of cervical length is not recommended in asymptomatic or otherwise low-risk women, at least in part because no intervention has been proven to improve maternal or fetal outcome in women in whom cervical ultrasound demonstrates an increased risk for preterm delivery. Cervical length measurement may be more useful in the evaluation and management of women who present with symptoms of preterm labor (eg, deciding if risk of preterm delivery is such that admission, tocolysis or treatment with betamethasone are appropriate). Other screening tools (eg, fetal fibronectin) and or physical examination may also be useful in evaluating such patients. (See "Tests for prediction of preterm labor and delivery").
EXAMINATION OF OTHER PELVIC ORGANS AND STRUCTURES — Ultrasound is a safe and effective imaging modality for evaluating abdominal organs during pregnancy. As an example, ovarian cysts that either predate pregnancy or are discovered at the time of an obstetrical ultrasound examination can be followed to exclude rapid growth or an appearance worrisome for malignancy. As another example, the myometrium can be examined to identify uterine leiomyoma in women with unusual pelvic pain or uterine size greater than that expected by menstrual dates [33]. In addition, abdominal ultrasound examination can be used to evaluate pregnant women for multiple medical or surgical problems such as appendicitis, renal obstruction, and gallbladder or liver disease and is generally preferable to studies using ionizing radiation. (See individual topic reviews).
ULTRASOUND AS AN ADJUNCT TO INVASIVE PROCEDURES — Real-time ultrasound imaging allows experienced practitioners to guide needles, catheters, and other instruments into the fetus or adjacent structures for diagnosis or treatment. Some examples of sonographically guided procedures are listed in the table (show table 4). (See individual topic reviews on these procedures).
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