Achondrogenesis is a condition characterized by malformations of cartilage, severely shortened limbs, a shortened trunk, and an abnormally large head. Achondrogenesis is the second most common lethal short-limb disorder, and it has an estimated incidence rate ranging between 1 in 40,000 and 1 in 50,000 live births. Achondrogenesis is estimated to be the cause of 1 in 650 perinatal deaths.
Achondrogenesis is divided into three subtypes- type IA, type IB, and type II- based on the severity of the shortened limbs and degree of ossification, or bone formation, of the skull and spine. All three types are associated with an increased risk of premature birth and stillbirth, with types IA and IB having a stillbirth rate of 50%. The rate of premature birth and stillbirth is lower with type II, and babies born with type II tend to have larger birth weights, longer limbs, and survive for a longer period of time after birth. Despite longer periods of survival post birth in cases of type II, all three types of achondrogenesis are fatal in the perinatal period.
Achondrogenesis is also associated with a distended abdomen and small chest as well as other diagnoses like cystic hygroma, polyhydramnios, underdeveloped lungs, and hydrops fetalis. Achondrogenesis is not associated with aneuploidy, or chromosomal abnormalities.
Types IA and IB are inherited as autosomal recessive conditions, meaning that the recurrence risk for this condition is 1 in 4, or 25%. The recurrence risk for type II, which is typically caused by a gene mutation, is almost non-existent; however, family members may be asymptomatic carriers for type II. Parental genetic testing is recommended prior to subsequent pregnancies.