Achondrogenesis is characterized by severe micromelia, macrocrania, and short trunk. Although rare, it is the second most common lethal skeletal dysplasia after thanatophoric dysplasia
Achondrogenesis is classified as type I (ACGIA, ACGIB) and type II forms (ACGII). Achondrogenesis type II, hypochondrogenesis, and neonatal spondyloepiphyseal dysplasia congenita are now known to be phenotypic variants of the same disorder.
Achondrogenesis results from mutations in the TRIP11 (ACGIA), SLC26A2 (ACGIB), and COL2A1 (ACGII) genes, with achondrogenesis types IA and IB being autosomal recessive conditions and achondrogenesis type II most often being sporadic. Prenatally, polyhydramnios and hydrops leading to fetal demise can be observed. Skeletal anomalies include severe micromelia, a short and narrow thorax with pulmonary hypoplasia, and absent or very abnormal ossification of the skull and vertebral bodies.