THE MIDTRIMESTER ANOMALY SCAN
As already mentioned, in most countries, the survey of the fetal anatomy needed to perform a screening of congenital anomalies is carried out in the 2nd trimester, usually between the 18th and 21st weeks of gestation. In this regard, the ideal time for the sonographic assessment of fetal anatomy should be at 24–26 weeks’ gestation, to give an optimal balance between the amount of amniotic fluid and the volume and developmental stage of the fetal organs. In most cases where the timing of the anomaly scan is shifted, this is due to the fact that in those countries where there is a legal time limit for termination of pregnancy, this is usually set at 22–24 weeks’ gestation, which represent the limit of fetal viability. Usually, the examination protocol includes: • a biometric assessment usually consisting of biparietal diameter, head circumference, abdominal circumference, and femur length • anatomic assessment that, depending on different national guidelines, consists of visualization of all or part of the following structures: – head: skull, cerebral hemispheres, falx, cerebellum and cisterna magna, lateral ventricles, orbits, lips, and facial profile – thorax: 4-chamber view (with or without outflows) and the lungs
– abdomen: stomach, liver, bowel, kidneys and bladder, and abdominal wall
– limbs: presence of the four limbs and of the extremities
– spine • evaluation of the fetal adnexa, placenta and amniotic fluid Pretest information.
Acoustic window impairment. The diagnostic accuracy of midtrimester screening for the detection of congenital anomalies will be illustrated below. Here, we wish to underline how important it is, for medicolegal reasons, to describe in the report and to express in the pre- and post-test counseling the existence of any factor that may reduce the diagnostic accuracy of the ultrasound examination. These include maternal and fetal limitations. Maternal causes of impaired acoustic window. The most important factor that may greatly reduce the diagnostic potential of a transabdominal ultrasound examination is the presence of maternal obesity, which, unfortunately, is becoming a real problem due to the increased prevalence of this condition in the populations of the developed countries. The impairment of the acoustic window exhibits a positive linear correlation with the thickness of the abdominal subcutaneous adipose tissue.3 It is common experience, however, that in some cases resolution and penetration are also significantly reduced in the absence of evident maternal overweight, probably due to individual differences in subcutaneous adipose tissue water and fat content. Another factor that may limit the diagnostic accuracy of the midtrimester scan is an increased tone or contracture of the abdominal musculature, usually due to maternal anxiety. The presence of striae rubrae from dysmetabolic conditions or of large abdominal scars or burns can also have a significant impact on the quality of the ultrasound examination.
Finally, the most frustrating condition to be confronted with in the course of an ultrasound examination is, in our experience, a previous abdominoplasty. In this case, several concurrent factors contribute to the limitation of diagnostic accuracy: the extensive cleavage of the whole abdominal subcutaneous tissue from the underlying muscular fascia associated with a long cutaneous surgical wound scar, residual abdominal fat, and a dramatic increase in abdominal firmness represent a frustrating although not insurmountable problem. Fetal causes of impaired acoustic window. The most common cause of (fortunately transient) impairment of the acoustic window is represented by an unfavorable fetal lie: an anterior spine, especially if associated with a transverse lie, makes assessment of the heart and the craniofacial area often impossible. However, in these cases, it is often sufficient to rescan the woman after 20–60 minutes to let the fetus change its position and remove the cause of the acoustic window impairment. Fetal crowding is also a potential cause of acoustic window impairment, with the degree of impairment increasing with the number of fetuses. Another factor that can significantly limit the diagnostic accuracy of the midtrimester scan is an abnormal amount of amniotic fluid. In particular, severe oligohydramnios, from premature rupture of membranes or renal anomalies, and severe polyhydramnios, from fetal anomalies, twin-to-twin-transfusion syndrome, or
Prior to the scan, the woman should be informed about the potential diagnostic accuracy of the examination, of its screening nature, and of the technical and practical limitations of the ultrasound examination. In addition, it is useful to inform the couple that the scan will not have a predefined duration, but that this depends on several factors, including acoustic window limitations (e.g. maternal obesity) and fetal lie.
Figure 1.3 Evolving (late-onset) malformations: a few examples of
evolving lesions are shown. (a) Microcephaly: compare the reduced
head area (<3 standard deviations) with the normal abdominal area.
(b) Aortic coarctation: the axial 3-vessel and trachea view shows the
severely hypoplastic aortic arch (tubular hypoplasia). (c) Left-sided
congenital diaphragmatic hernia: the presence in the thorax of the
stomach is evident. (d) Jejunal atresia: extremely severe dilatation of
jejunal loops. (e) Aneurysm of the vein of Galen: on the transventricular
view, the huge midline vascular lake, consistent with the diagnosis
of aneurysm of the vein of Galen, is highlighted on power
idiopathic, can be responsible for an impairment of the acoustic window for different reasons: in oligohydramnios, the natural contrast agent represented by the amniotic fluid is absent and, in addition, the limbs are often adducted, and these two factors usually limit the assessment of the fetal limbs and heart. With severe polyhydramnios, the increased fetal movements and the significant increase in the distance between the transducer and the fetal body are the two main limitations. The abdominal acoustic windows. It is useful to know that the normal anatomy of the human abdominal wall provides a few preferential ‘channels’ to exploit in the case of an impaired acoustic window. These preferential points of access are characterized by a reduced amount of subcutaneous adipose tissue with a consequently reduced transducer–fetus median distance. Two of these anatomic regions are the periumbilical area, where there is virtually no adipose tissue and which may be exploited if no air is left between the transducer and the actual maternal umbilicus, and the lateral regions of the abdomen (i.e. iliac fossae), where the mean thickness of the abdominal adipose tissue is less than in the hypogastric area (between the symphysis and the umbilical area). To take advantage of this type of approach, the patient may be asked to roll onto one side (which one depends on the position of the fetus) in order to better expose the lateral abdominal area to the operator. Often, using this type of approach, the increased muscular resistance offered by some patients (see above) also tends to be reduced. A third preferential point of access is the suprapubic area/fold. In general, at 20–22 weeks with the fetus in the vertex position, the fetal head may be approached through the suprapubic area, while the craniofacial anatomy may be assessed through the periumbilical area. A useful hint may be to use the maternal bladder as a wedge: a full bladder will push the uterus (and the fetus) upwards, towards the umbilical area; on the contrary, an empty bladder may allow the fetal head to descend into the pelvis, where it can be explored through the suprapubic window. The 3rd-trimester examination. This ultrasound scan usually aims at the recognition of growth restriction. A second endpoint is the detection of late-onset congenital anomalies. Evolving and late-onset malformations (Figure 1.3 and Table 1.3). The term ‘evolving’ may indicate two conditions: (i) that a malformation may potentially arise late
in gestation (late onset), because it is due to an arrest in the development/growth of a given anatomic structure (microcephaly or semilunar valve stenosis); or (ii) that it is already present at midtrimester but has the potential to progress toward more severe stages in the 3rd trimester (diaphragmatic hernia, semilunar valve stenoses, or upper renal obstructive lesions). As a result, the midtrimester anomaly can be unremarkable in all conditions covered by the former definition. A partial list of the most common evolving lesions is given in Table 1.3.