In all of these dysplasias, there is abnormal endochondral ossification, but periosteal ossification is not affected.
Heterozygous achondroplasia is the most common nonlethal skeletal dysplasia.
The distinctive clinical and radiological features allow a precise diagnosis, as there is little variability in the appearance of affected patients. There is also a very evident molecular homogeneity. On histopathology of the growth plate, there is a quantitative decrease in endochondral ossification. Precise prenatal ultrasonographic diagnosis is possible in the third trimester, and sometimes even in the second. Hypochondroplasia is a relatively common, milder form of achondroplasia, which varies within and between families and lacks the neurological complications often seen in achondroplasia of this group.