**8. Types of limb abnormalities**

The most common types of limb anomalies include abnormal number of digits (the higher frequency having polydactyly), abnormal hand/foot position, limb reduction defects, and arthrogryposis. It seems that unilateral limb defects are rarer.

Abnormal hand position is defined as clenched hands or overlapping digits. Arthrogryposis is defined as fetal joint contractures and rigidity.

Counting the fingers is not part of routine scan. Polydactyly is more common in some ethnic groups, such as African Americans. It may occur isolated or may affect both hands and feet.

In amniotic band syndrome, fingers can be missing. This is due to the arrest in development and not as a primary defect in the blastulation process.

In some rare syndromes such as ectrodactyly-ectodermal dysplasia-cleft syndrome (EEC syndrome), missing fingers occur in association with complex malformations. EEC syndrome is a rare form of ectodermal dysplasia. It is an autosomal dominant disorder, inherited as a genetic trait. EEC includes also vesicoureteral reflux, recurrent urinary tract infections, obstruction of the nasolacrimal duct, decreased pigmentation of the hair and skin, missing or abnormal teeth, enamel hypoplasia, absent punctae in the lower eyelids, and photophobia. Occasional, cognitive impairment, kidney anomalies, and conductive hearing loss may appear.

In the development of fetal limbs the free movement itself has a very important role. This is favored by the proximity of fluid in the uterine cavity. The limbs should move freely within each joint. Normal movement assures the normal positions of the hip, knee, elbow, ankle, and wrist joints.

Apparition of abnormal angulation of the ankle joint (ankle clubbing, talipes equinovarus) is frequent, with a prevalence of 1 in 100 live births. The best image is to get a coronal section of the ankle, in which the extended ankle straight along is seen, in a normal spatial relationship with the lower leg. In ankle clubbing, the ankle deviates medially. In the third trimester, especially when the amniotic fluid decreases, a slight subjective angulation of the ankle is common. The key feature for ruling out a true clubbing is the normal shape of the foot. Unilateral ankle clubbing is usually an isolated defect. Bilateral clubbing should be investigated for chromosomal anomalies and genetic syndromes.

The wrist is very flexible and the position of the fingers is also variable. Thus, the examination can find them in a wide variety of positions. In late second trimester and third trimester, the resting position is fisting. The hand can be stimulated to open, showing all four fingers and the thumb. Due to the anatomy of the hand, the thumb is visible in a different plane from the rest of the fingers. Due to this particular context, the diagnosis of abnormal hand position is more difficult than in distal limb.
