What is the primary characteristic of positional plagiocephaly?

The primary characteristic of positional plagiocephaly is a flattened area on the head. This condition arises when an infant's head develops a flat spot due to prolonged pressure on one area, often from lying in the same position for extended periods. As the skull of infants is still soft and pliable, the repeated pressure can lead to asymmetry and distortion in the shape of the head.

In the context of this condition, the flattened area typically appears on one side of the skull, resulting in noticeable asymmetry when viewed from the front or side. This characteristic is pivotal for identifying and diagnosing positional plagiocephaly, and it differentiates this condition from normal variations in head shape.

The other options do not accurately represent the defining features of positional plagiocephaly. For instance, a rounded forehead is not a distinguishing characteristic of this condition. Similarly, excessive hair growth and visible signs of stress are unrelated to the structural changes seen in the head shape of infants with positional plagiocephaly. Therefore, the identification of a flattened area on the head is critical for recognizing and addressing this common infant condition.