Distal obstruction of the urinary tract is more frequent in male fetuses, and the most common etiology is posterior urethral valves. Characteristically, the bladder and proximal urethra are dilated, termed the "keyhole" sign, and the bladder wall is thick (Fig. 10-45). Oligohydramnios, particularly before midpregnancy, portends a poor prognosis because of pulmonary hypoplasia. Unfortunately, the outcome may be poor even with normal amnionic fluid volume. Evaluation includes a careful search for associated anomalies, which may occur in 40 percent of cases, and for aneuploidy, which has been reported in 5 to 8 percent (Hayden, 1988; Hobbins, 1984; Mann, 2010). If neither are present, afected male fetuses with severe oligohydramnios who have fetal urinary electrolytes suggesting a potentially favorable prognosis may be fetal therapy candidates. Evaluation and treatment of fetal bladder outlet obstruction is discussed in Chapter 16 (p. 325).
The 2015 revision of the Nosology and Classification of Genetic Skeletal Disorders includes an impressive 436 skeletal anomalies in 42 groups, characterized by genetic abnormalities, phenotypic features, or radiographic criteria (Bonafe, 2015). he two types of skeletal dysplasias are osteochondrodyspasiasthe generalized abnormal development of bone and/or cartilage, and dysostoses-which are abnormalities of individual bones, for example, poydacyy. In addition to these maormations, skeletal abnormalities include dormations, as with some cases of clubfoot, and disruptions such as limb-reduction defects.
The prevalence of skeletal dysplasias approximates 3 in 10,000 births. Two groups account for more than half of all cases: the ibroblast growth octor 3 (FGF3) chondrodysplasia group and the osteogenesis impeecta and decreased bone density group. Each occurs in 0.8 in 10,000 births (Stevenson, 2012).
Evaluation of a pregnancy with suspected skeletal dysplasia includes a survey of every long bone, as well as the hands and feet, skull size and shape, clavicles, scapulae, thorax, and spine. Reference tables are used to determine which long bones are afected and ascertain the degree of shortening (Appendix, p. 1267). Involvement of all long bones is termed micromelia, whereas predominant involvement of only the proximal, intermediate, or distal long bone segments is termed rhizomelia, meso melia, and acromelia, respectively. he degree of ossiication should be noted, as should presence of bowing or fractures. Each of these may provide clues to narrow the diferential diagnosis and occasionally suggest a specific skeletal dysplasia. Many, if not most, skeletal dysplasias have a genetic component, and knowledge of specific mutations has advanced dramatically (Bonafe, 2015).
nosis, it is frequently possible to determine whether a skeletal dysplasia is lethal. Lethal dysplasias show profound long bone shortening, with measurements <5th percentile, and display cent (Nelson, 2014; Rahemtullah, 1997; Ramus, 1998). Gen erally, other sonographic abnormalities are evident. Pulmonary hypoplasia is suggested by a thoracic circumference < 80 per cent of the abdominal circumference value, by a thoracic cir cumference <2.5th percentile, and a cardiac circumference > 50 percent of the thoracic circumference value (Appendix, p. 1266). Afected pregnancies also may develop hydramnios and/or hydrops (Nelson, 2014).