The diagnosis of Rett syndrome is currently determined by a series of criteria, the first three of which are concerned with the initial period of normality. Using the Australian Rett Syndrome Database, information about this early period was obtained by questionnaires to paediatricians and families. For girls born between 37 and 42 weeks' gestation, 70.7% of girls with Rett syndrome had birthweights <3500g compared with 62.2% of singleton births of the same gestational age in Western Australia between 1980 and 1992. The mean head circumference at birth for infants with Rett syndrome was lower than the mean head circumference at birth for the reference populations after accounting for gestational age. Forty-one percent of parents reported that their daughter had some perinatal difficulty, while in 22.4% of cases clinicians reported some perinatal abnormality. In 46.5% of girls, parents reported that their daughter's development or behaviour had been unusual in the first 6 months. Our results provide further evidence that the girl with Rett syndrome may not be normal at birth.