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Ultrasound in obstetrics and gynecology uses high-frequency energy (2-7 million cycles per second) that is reflected from solid and fluid structures. The information obtained from insonation is translated into an interpretable image (i.e., two-dimensional, M-mode, Doppler ultrasound) or sound (i.e., Doppler ultrasound). The physics of ultrasound were described by scientists in the late 1800s and early 1900s. Ultrasound was first used in 1916 to 1917 to detect U-boats; in fact, a military (and early) term for ultrasound is sonar, which stands for sound navigation and ranging. (1) Sokolov used ultrasound to detect flaws in metal in 1929. (2) The engineering techniques involved in ultrasound application were classified until after World War II; medical ultrasound was developed after declassification. Advances in instrumentation have resulted in ultrasound s becoming an integral part of obstetric care.
Two-dimensional and M-mode ultrasound are produced by electronic stimulation of a piezoelectric crystal. The ultrasound energy that is generated is reflected from interfaces between tissue planes and translated into images. The depth of focus varies with different instruments; in general, as the frequency (i.e., in megahertz) of the transducer increases, the resolution increases and the penetration of the ultrasound decreases.
Two-dimensional ultrasound produces an image of a plane through the tissues sampled (Fig. 14-1) . The major obstetric applications of two-dimensional ultrasound have been for fetal measurement, fetal anatomic survey, and determination of fetal condition.
M-mode echocardiography supplies high-resolution information about one series of points over time. M-mode ultrasound depicts a single portion of the linear or sector display and plots it over time, resulting in a wavy pattern in the presence of movement (Fig. 14-2) . Its primary use when initially included on ultrasound machines used for obstetric examination was to document fetal cardiac motion. Since its introduction, M-mode ultrasound has been used to study fetal cardiac arrhythmias, to document pericardial effusions, and to measure cardiac wall, vessel, and chamber dimensions.
Doppler ultrasound detects a change in frequency of reflected ultrasound returning from a moving target and converts this information into a visible image by way of fast Fourier transformation. It is used to measure the velocity of blood flow through the fetal and maternal cardiovascular systems (Fig. 14-3) (Figure Not Available) . Continuous-wave Doppler instruments sample all moving targets within the beam path, since information is continually transmitted and received. Pulsed Doppler is used to sample discrete areas of fetal or maternal anatomy; because signals are intermittently transmitted and received, distance from the transducer can be calculated as a function of time. Umbilical artery Doppler velocity waveforms are used to assess features of the umbilical-placental circulation, including the relative amount of diastolic flow. Doppler waveforms from the maternal parauterine regions have been used to predict pregnancy outcome, although there is some controversy about the significance of parauterine signals. Pulsed Doppler techniques are used to confirm the presence of fetal cardiac abnormalities; to study intracardiac regurgitation, stenoses, and shunts; to measure fetal cardiac
Figure 14-1 Two-dimensional ultrasound image of fetal fingers juxtaposed to
a fetal profile.
Figure 14-2 Two-dimensional and M-mode ultrasound of a fetus with complete heart
block. The cursor crosses from left ventricle to right atrium. Right atrial
rate is 133 beats per minute (black arrow);
ventricular rate is 53 beats per minute (white arrow).
Figure 14-3 (Figure Not Available) Doppler velocity waveforms
through a normal fetal heart. (A) Tricuspid
valve. (B) Mitral valve. (C) Pulmonary valve. (D) Aortic
valve. The atrioventricular valve waveforms have two peaks, whereas the semilunar
valve waveforms have one peak. (From Shenker L, Reed KL, Marx GR,
et al. Fetal cardiac Doppler flow studies in prenatal diagnosis of heart
disease. Am J Obstet Gynecol 1988; 158:1267.)
The major types of ultrasound transducers are listed in Table 14-1 . The linear-array transducer uses a series of crystals arranged in a linear manner, with the ultimate product being a summation of information derived from the individual crystals. Sector scanning, which samples a larger area of anatomy from a small contact zone, may be done either mechanically, with one crystal being rotated, or electronically by varying the time of electrical excitation of individual crystals. Instruments that allow electronic focusing are called phased-array transducers. Crystals may also be oriented in an annular manner, with each concentric arrangement of crystals focused at a different depth. Instruments may be focused dynamically on the basis of returning ultrasound information. Computer manipulation of ultrasound images has permitted high-resolution images of larger regions of fetal or maternal anatomy to be created by simultaneously varying focal zones.
The three primary applications of ultrasound in obstetrics are to measure the fetus, examine the fetal anatomy, and establish fetal viability or well-being. In addition, placental location and condition are assessed as well as the amount of amniotic fluid. Fetal position is recorded. If more than one fetus is present, a similar assessment is made for both fetuses (Fig. 14-4) .
Aspects of fetal behavior assessment have been quantitated; the fetal biophysical profile is used for prenatal testing. Ultrasound evaluation of the physiologic status of the fetus is in the experimental stage. Umbilical artery Doppler flow velocity waveforms are the best example of this kind of information.
The question of whether all fetuses should undergo ultrasound study during gestation was answered in the negative by a National Institutes of
| Linear array |
| Sector |
| Mechanical (crystal rotated) |
| Electronically focused (phased) |
| Curvilinear: linear with smaller contact area |
| Vaginal: usually similar to sector scanners |
Figure 14-4 Two-dimensional ultrasound of a twin gestation. One twin is seen
in profile, the other as a cross section of the abdomen.
Women who previously have had a complicated pregnancy are candidates for ultrasound examination during subsequent pregnancies. The studies may include a dating ultrasound to establish gestational age as early as possible, a scan for fetal anomalies to rule out recurrence, and serial scanning to document fetal growth. Fetal growth retardation recurs in 23% of women with a positive history (4) and premature labor recurs in 34% of such women. (5)
Even without previous pregnancy experience, women with certain maternal conditions can be identified at the beginning of pregnancy to be at risk for having a problem with the fetus. The fetuses of women with insulin-dependent diabetes are three to four times more likely to have congenital anomalies. The fetuses of women with connective tissue disease may have congenital heart block, cardiac anomalies, and growth retardation. Isoimmunization may result
| Estimation of gestational age for patients with uncertain clinical dates |
| Evaluation of fetal growth |
| Vaginal bleeding of undetermined cause in pregnancy |
| Determination of fetal presentation |
| Suspected multiple gestation |
| Adjunct to amniocentesis |
| Significant uterine size-date discrepancy |
| Pelvic mass |
| Suspected hydatidiform mole |
| Suspected polyhydramnios or oligohydramnios |
| Suspected abruptio placentae |
| External version |
| Estimation of fetal weight with premature rupture of membranes or premature labor |
| Abnormal serum alpha-fetoprotein |
| Follow-up an identified fetal anomaly |
| Follow-up placental location with identified placenta previa |
| History of congenital anomaly |
| Serial evaluation of fetal growth in multiple gestation |
| Adjunct to cerclage |
| Suspected ectopic pregnancy |
| Special procedures |
| Suspected fetal death |
| Suspected uterine abnormality |
| Intrauterine device localization |
| Ovarian follicle development surveillance |
| Biophysical evaluation after 28 wk |
| Observation of intrapartum events |
| Evaluation of fetal condition in late registrants for prenatal care |
| From National Institute of Child Health and Development. Diagnostic ultrasound imaging in pregnancy. Report of the Consensus Development Conference. NIH Publ. No. 84-667. Washington, DC: Government Printing Office, 1984. |
Drug use during pregnancy may be indicated for maternal health, inadvertent, or intentional but recreational. The use of diphenylhydantoin may result in fetal craniofacial malformations or cardiac or skeletal defects. Lithium ingestion has been associated with the development of Ebstein s anomaly in some exposed fetuses. Isotretinoin use may result in spontaneous abortion or fetal anomalies such as hydrocephalus, microcephaly, or ear or cardiac defects. Alcohol use has been reported to cause fetal craniofacial malformations, cardiac defects, and joint abnormalities as well as intrauterine growth retardation. Viruses such as rubella, toxoplasmosis, cytomegalovirus, and parvovirus may affect the fetus adversely.
If the pregnancy does not progress normally, an ultrasound examination may be appropriate. Fundal height abnormalities resulting from inadequate fetal growth or amniotic fluid volume may indicate that growth retardation or an anomaly such as renal agenesis is present. A fundus that is large for dates may indicate a macrosomic fetus or increased amniotic fluid. Polyhydramnios may result from such problems as fetal swallowing or intestinal abnormalities. An increased fundal height may also suggest multiple gestation; twin fetuses have an increased risk of congenital abnormalities and, if monochorionic, twin-twin transfusion. Ultrasound studies that indicate anatomic abnormalities require careful follow-up studies for the presence of associated abnormalities. Fetal hydrops may result from a number of conditions but most commonly is associated with fetal anemia or cardiac abnormalities of anatomy, rhythm, or function. If preterm labor occurs, the fetus should be examined for estimation of size and the presence of anomalies. Vaginal bleeding may result from placental abnormalities or indicate the presence of an
| History of Anomalies |
| Growth retardation |
| Premature labor |
| Maternal Conditions Predisposing to Anomalies |
| Diabetes, insulin dependent |
| Connective tissue disease |
| Isoimmunization |
| Phenylketonuria |
| Age |
| Maternal Conditions Predisposing to Growth Retardation |
| Hypertension |
| Vascular disease |
| Cyanosis |
| Exposure to Drugs |
| Diphenylhydantoin |
| Lithium |
| Isotretinoin |
| Alcohol |
| Exposure to Viruses |
| Rubella |
| Toxoplasmosis |
| Cytomegalovirus |
| Parvovirus |
| Abnormal Pregnancy Progression |
| Abnormal growth |
| Abnormal amniotic fluid volume |
| Multiple gestation |
| Abnormal ultrasound |
| Anomalies |
| Hydrops |
| Premature labor |
| Bleeding |
| Persistent malpresentation |
| Decreased fetal movement |
| Fetal arrhythmia |
| First Trimester | Second Trimester | Third Trimester |
|---|---|---|
| Pregnancy location | Fetal size for dates | Fetal size for growth |
| Size of fetus and sac | Detailed fetal anatomy | Fetal anatomy |
| Pregnancy viability |
|
Biophysical behavior |
| Gross anatomy |
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|
| Limits: fetal size (small) | Limits: fetal size (small); fetal age (early) | Limits: fetal size (large); fetal position |
Information obtained from the ultrasound study varies, based on the size of the pregnancy (Table 14-4) and the clinical indication for the study.
Early-pregnancy studies have traditionally focused on three major determinations. First, establishing the location of the pregnancy is paramount for ruling out an ectopic pregnancy. Second, fetal age is estimated by crown-rump length or gestational sac mean
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Second-trimester examinations usually are performed to determine the fetus size and anatomy. Although many potential anatomic anomalies may be identified during a second-trimester examination, some problems (e.g., hydrocephalus, microcephaly, achondroplasia, certain cardiac defects) may not become evident until later in the pregnancy or after birth.
Third-trimester examinations focus more on size for growth considerations than on size for age considerations. Anatomy, if not yet carefully studied, should also be included. In the third trimester, the additional examination of fetal biophysical behavior is performed as part of prenatal testing of fetal well-being. Fetal size and position may limit the amount of information gathered about anatomy in the third trimester.
In each scan, certain features should be examined (Table 14-5) . These include the number of fetuses,
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