Prevalence:

• 1 in 4,000 births.
• 25% are stillborn and 30% die in the neonatal period.
  

Approach to prenatal diagnosis:

• There is a wide range of rare skeletal dyplasias, each with a specific recurrence risk, dysmorphic expression, and implications for neonatal survival and quality of life.
• The incidental discovery of a skeletal dysplasia on routine ultrasound screening, in a pregnancy not known to be at risk of a specific syndrome, necessitates a systematic examination of the limbs, head, thorax and spine to arrive at the correct diagnosis.

Assessment of long bones:

• Shortening of the extremities can involve the entire limb (micromelia), the humerus or femur (rhizomelia), the radius, ulna, tibia or fibula (mesomelia) or the hands and feet (acromelia). The femur is abnormally short even in mesomelic dwarfism and, therefore, in routine fetal abnormality screening the femur is measured and compared subjectively to all long bones. Severe limb reductions, as in thanatophoric dwarfism and achondrogenesis can be detected from 16 weeks’ gestation, whereas in achondroplasia limb shortening becomes obvious >22 weeks.
• Abnormal shape. In some conditions there is pronounced bowing (e.g. campomelic dysplasia, thanatophoric dwarfism), and in others fractures and callus formation may also be detected (e.g. osteogenesis imperfecta, achondrogenesis and hypophosphatasia).
• Reduced echogenicity of bones due to hypomineralization is seen in some disorders (e.g. hypophosphatasia, osteogenesis imperfecta and achondrogenesis). Virtual absence of ossification of the spine in achondrogenesis, may lead to the erroneous diagnosis of complete spinal agenesis. Poor mineralization of the skull in hypophosphatasia, may result in the misdiagnosis of hydrocephalus.
• Absence of extremities, such as amelia (complete absence of extremities), acheiria (absence of the hand), phocomelia (seal limb) or aplasia–hypoplasia of the radius or ulna, are often inherited as part of a genetic syndrome (Holt–Oram syndrome, thrombocytopenia with absent radii syndrome). Another cause of focal limb loss is the amniotic band syndrome.

 Evaluation of hands and feet:

• Several skeletal dysplasias are associated with alterations of the hands and feet.
  • Polydactyly: presence of more than five digits. It is classified as postaxial if the extra digits are on the ulnar or fibular side and preaxial if they are located on the radial or tibial side.
  • Syndactyly: soft tissue or bony fusion of adjacent digits.
  • Clinodactyly: deviation of a finger(s).
  • Disproportion between hands and feet and the other parts of the extremity may also be a sign of a skeletal dysplasia.

Examination of fetal movements:

• Arthrogryposis and multiple pterygium syndrome are characterized by limitation of flexion or extension of the limbs.

Evaluation of the fetal thorax:

• Several skeletal dysplasias are associated with a small thorax, which leads to pulmonary hypoplasia and neonatal death. The diagnosis of a small thorax can be made by examining the thoracic-to-abdominal circumference ratio or the thoracic-to-head circumference ratio. The thoracic circumference is measured at the level of the four-chamber view of the heart.

Evaluation of the fetal head:

• Several skeletal dysplasias are associated with reduced ossification of the skull bones. The face should also be examined for the diagnosis of hypertelorism, micrognathia, short upper lip, and abnormalities of the ears.

Diagnostic tests complementary to sonography:

• Prenatal or postnatal evaluation includes DNA analysis for an increasing number of skeletal dysplasia. Some can now be diagnosed by cfDNA testing of maternal blood. Postnatally, examination of skeletal radiographs is of particular importance, since the classification of skeletal dysplasias is largely based on radiographic findings.