The primary causes of infant hair loss - especially in patches at the back of the head or a "monk's ring," that don't require medical treatment are:

Rubbing
Babies from 3-6 months of age commonly rub off a patch of hair on the back of their head due to friction during head-turning against the mattresses of cribs, playpens, and infant seats. The hair grows back once they start sitting up. *

Other possible causes:

Newborn Hair loss
The hair of many newborns falls out during the first few months of life. This baby hair is replaced by permanent hair. *

Stress
Hair follicles are very sensitive to stress. The hair begins to fall out about 3 months (100 days) after a severe stress (such as high fever, severe illness, a psychological crisis, a crash diet, surgery or even childbirth.) The hair falls out from all parts of the head over the next 3 or 4 months. After the hair stops shedding, it takes another 6 to 8 months for all of the hair to grow back. The whole cycle takes about 12 months. This type of hair loss is called telogen effluvium*.

Call your child's health care provider if your child's hair loss is caused by something other than rubbing or friction.