The impact of maternal age at menarche on newborn size was tested using data from 4,996 single births taking place at Vienna, Austria, between 1985 and 1995--so-called teenager pregnancies were excluded from the present analyses. All women experienced pregnancies between the ages of 19 to 42 years. Maturational time was related significantly to infant weight and length independent of maternal age and behavioral variables such as smoking. Early maturation, i.e., age at menarche before the 12th birthday, was significantly associated with decreased newborn weight and size. The incidence of low-weight newborns was significantly higher in early-maturing mothers. The more favorable nutritional status of women whose menarche occurred relatively early was not able to compensate for the negative effects of early maturation on intrauterine growth. Higher circulating estrogen levels in early maturers preserved into adulthood are discussed as possible reasons for intrauterine growth retardation of the offspring of early-maturing mothers.