Achondroplasia is a disorder of bone growth that causes the most common type of dwarfism.

Achondroplasia is one of a group of disorders called chondrodystrophies or osteochondrodysplasias.

Achondroplasia may be inherited as an autosomal dominant trait, which means that if a child gets the defective gene from one parent, the child will have the disorder. If one parent has achondroplasia, the infant has a 50% chance of inheriting the disorder. If both parents have the condition, the infant's chances of being affected increase to 75%.

However, most cases appear as spontaneous mutations. This means that two parents without achondroplasia may give birth to a baby with the condition.

The typical appearance of achondroplastic dwarfism can be seen at birth. Symptoms may include:

• Abnormal hand appearance with persistent space between the long and ring fingers
• Bowed legs
• Decreased muscle tone
• Disproportionately large head-to-body size difference
• Prominent forehead (frontal bossing)
• Shortened arms and legs (especially the upper arm and thigh)
• Short stature (significantly below the average height for a person of the same age and sex)
• Spinal stenosis
• Spine curvatures called kyphosis and lordosis

During pregnancy, a prenatal ultrasound may show excessive amniotic fluid surrounding the unborn infant.

Examination of the infant after birth shows increased front-to-back head size. There may be signs of hydrocephalus ("water on the brain").

X-rays of the long bones can reveal achondroplasia in the newborn.