DSM IV Criteria for Rett's Disorder^*

A. All of the following:

1. Apparently normal prenatal and perinatal development.
2. Apparently normal psychomotor development through the first 5 months after birth.
3. Normal head circumference at birth.

B. Onset of all of the following after the period of normal development:

1. Deceleration of head growth between ages 5 and 48 months.
2. Loss of previously acquired purposeful hand skills between ages 5 and 30 months with subsequent development of stereotyped hand movements (e.g. hand-wringing or hand washing).
3. Loss of social engagement early in the course (although often social interaction develops later).
4. Appearance of poorly coordinated gait or trunk movements.
5. Severely impaired expressive and receptive language development with severe psychomotor retardation.

Reference