From the Centers for Disease Control :
"What is microcephaly?
Microcephaly is a condition where a baby’s head is much smaller than expected. During pregnancy, a baby’s head grows because the baby’s brain grows. Microcephaly can occur because a baby’s brain has not developed properly during pregnancy or has stopped growing after birth, which results in a smaller head size. Microcephaly can be an isolated condition, meaning that it can occur with no other major birth defects, or it can occur in combination with other major birth defects."
CDC, Facts about Microcephaly