Routine screening for fetal anomalies: expectations
Section snippets
Epidemiology of fetal anomalies
It is important to have an understanding of the epidemiology of birth defects for several reasons. First, in examining the literature relating to the detection of birth defects by ultrasound, it is worthwhile to be aware of what the overall incidence of fetal anomalies is in the studied populations. If the incidence is lower than that expected in the population, complete ascertainment of anomalies may not have occurred. If higher, the detection rates are higher because the population may be
Sensitivity of ultrasound in detecting anomalies
Significant controversy exists regarding the sensitivity of ultrasound in detecting fetal anomalies. A comprehensive review of the existing studies relating to this issue has been recently published by Levi [2]. Levi [2] compiled data from 36 studies involving over 900,000 fetuses. The overall sensitivity for detecting fetal anomalies was 40.4% (range 13.3% to 82.4%). It is useful to examine two of these studies in detail. The RADIUS trial is important because it is used as an example by many
Timing of ultrasound
Most ultrasound laboratories recommend performing a fetal anomaly detection scan at 18 to 22 weeks of gestation. This recommendation has been based on very little data and primarily stems from subjective observations that this seems like a good time to visualize fetal anatomy at a time when termination of pregnancy is still an option. In a prospective study, Schwärzler et al [4] evaluated performing ultrasound screening for abnormalities at 18, 20, or 22 weeks. These investigators found that
Standards
Varying standards exist for the performance of a screening ultrasound examination. One of the most widely referred to in the United States is the set of standards published by the American Institute of Ultrasound in Medicine in 1994 [7]. In the section on fetal anatomy, these standards include examination of the following areas of fetal anatomy: cerebral ventricles; posterior fossa (including cerebellar hemispheres and cisterna magna); four-chamber view of the heart (including its position
Head and brain
Examination of the fetal head involves visualization of several key structures. The first of these structures are the lateral ventricles. The lateral ventricles are imaged in an axial view at the level of the atrium. The choroid plexus is visualized within the lateral ventricles and in cases of ventriculomegaly may be seen to be “dangling” (Fig. 1) [8]. Various measurements and ratios have been proposed to measure the lateral ventricle to assess for the presence of ventriculomegaly. The most
Adverse effects of ultrasound screening
Ultrasound is a noninvasive screening method that has been shown to be biologically safe in long-term studies [38], [39]. The adverse effects of ultrasound relate to the sensitivity and specificity of the ultrasound examination and the anxiety that this causes to the patient. It is important to make clear to the patient that ultrasound has its limitations. The patient must realize that not all malformations can be detected. The sonographer should also be aware of what is said to the patient.
Summary
Ultrasound has become a routine part of prenatal care. Despite this, the sensitivity and specificity of the procedure is unclear to many patients and health care providers. In a small study from Canada, 54.9% of women reported that they had received no information about ultrasound before their examination [40]. In addition, 37.2% of women indicated that they were unaware of any fetal problems that ultrasound could not detect [40].
Most centers that perform ultrasound do not have their own
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