This study was conducted to examine the effect of a number of variables related to the mother (age, educational level, employment status, cigarette smoking during pregnancy) and to the baby (sex and birth order) on newborns' birthweight. The research was carried out in the province of Istanbul, one of Turkey's large metropolises, at the Mother-Child Health and Family Planning Center in the district of Yenibosna, where 0-1 month old infants had been brought in for phenylketonuria screening and vaccinations. The mean age of the mothers was 26.1 ± 4.8 years and the mean birthweight of the newborns was 3236.7 ± 542.2 g. Of the infants, 6.3% were classified as having a low birthweight and 8.3% were large infants. The mean birthweight showed a statistical significance depending upon the infants' sex and birth order. The mean birthweight of the infants of the working mothers, as opposed to the unemployed mothers and the mothers in nuclear families, as opposed to the mothers in extended families, was higher.