Achondroplasia is a genetic condition affecting bone growth, resulting in the most common form of disproportionate short stature. It is caused by a mutation in the fibroblast growth factor receptor 3 (FGFR3) gene, which slows the conversion of cartilage to bone in the growth plates. While diagnosed after birth through physical examination, prenatal ultrasound plays a part in its initial detection. Ultrasound can identify physical markers that suggest the condition, allowing for further evaluation.
Key Ultrasound Indicators
During a pregnancy ultrasound, sonographers look for physical markers that suggest achondroplasia. The primary indicator is rhizomelic shortening, a significant shortening of the long bones where the upper arms (humerus) and thighs (femur) are disproportionately shorter than the forearms and lower legs. These measurements are compared to standard fetal growth charts to identify deviations.
Another indicator is an unusually large head (macrocephaly), often accompanied by a prominent forehead, known as frontal bossing. The facial profile may show midface hypoplasia, where the central part of the face appears underdeveloped. This can result in a flattened or depressed nasal bridge.
A more subtle marker is the “trident hand,” where an unusually wide space between the middle and ring fingers gives the hand a three-pronged appearance. While one sign alone is not conclusive, the presence of several indicators together strengthens the suspicion of achondroplasia.
Timeline of Prenatal Detection
Understanding when the signs of achondroplasia become visible on an ultrasound is important for detection. The routine anatomy scan around 20 weeks of gestation may appear normal, as the physical traits do not become apparent until later in the pregnancy.
The shortening of the limbs and other markers emerge in the late second or third trimester. These signs are often first suspected after 22 weeks and become more identifiable after 26 weeks of gestation. The femur, for instance, may follow a normal growth trajectory early on, only to slow down significantly as the pregnancy progresses into the third trimester.
Because of this delayed presentation, a normal 20-week scan does not rule out achondroplasia. If there is a reason to observe fetal growth more closely, regular monitoring, sometimes with scans every four weeks, may be recommended.
Diagnostic Certainty and Limitations
While ultrasound is useful for identifying the physical characteristics of achondroplasia, it has limitations. The findings provide a strong basis for suspicion but do not constitute a definitive diagnosis on their own. The accuracy of a suspected diagnosis increases when multiple indicators, such as limb shortening and specific facial features, are observed together.
The signs of achondroplasia can overlap with other rare skeletal dysplasias. A physician will consider a differential diagnosis, which involves evaluating other conditions that could cause similar skeletal abnormalities. Ultrasound serves as a screening tool that highlights the need for further testing.
Confirmatory Diagnostic Procedures
When ultrasound findings suggest achondroplasia, genetic testing is the next step to confirm the diagnosis. This involves analyzing fetal DNA for the specific mutation in the FGFR3 gene known to cause the condition. A sample of fetal DNA is obtained through one of two established procedures.
The most common method is amniocentesis, performed after 15 weeks of pregnancy. This procedure involves withdrawing a small amount of amniotic fluid, which contains fetal cells, through a thin needle. These cells are then analyzed in a lab to detect the FGFR3 gene mutation.
Another option is chorionic villus sampling (CVS), which can be performed between 10 and 13 weeks by taking a small tissue sample from the placenta. While it provides the same genetic information, CVS is less common for confirming achondroplasia because the physical signs are detected later in pregnancy.