However, since the parent does not have any extra or missing chromosome material, they are said to have a balanced translocation and they are usually normal and healthy. Rarely, mosaic trisomy 18 or 13 may occur when the error in cell division occurs after fertilization. These persons have some cells with an extra chromosome 18 or 13 and others with the normal number. Babies with trisomy 18 Babies with trisomy 18 appear thin and frail. They fail to thrive and have problems feeding.
Ears are usually low set on the head.
At birth, these babies are small for their age, even when delivered full-term, and have a weak cry. Their response to sound is decreased and there is often a history of infrequent fetal activity during the pregnancy. About 90 percent of babies with trisomy 18 have heart defects. They clench their fists in a characteristic manner and extending the fingers fully is difficult. The feet may be referred to as rocker bottom due to their shape.