Background : Low birth weight (LBW) is associated with high mortality and morbidity in neonates. The incidence of LBW varies between societies. The aim of this study was to determine the prevalence and risk factors of LBW infants in Mahdieh Maternity Hospital, in Tehran during an academic year.

Materials and methods : In this study all the LBW infants who were delivered alive in Mahdieh Maternity Hospital in 1996 were investigated. Initial data including maternal age, gestational age, parity, prenatal care, mode of delivery, sex, apgar score, cause of admission and death were carefully recorded from medical records.

Results : The incidence of LBW was 7.05%. More than half of the LBW babies were premature. The girls to boys LBW ratio was 1.3: 1. In the premature group the mother’s age range was 16-20 years whereas in the intra uterine growth retardation group (IUGR) was 21-25 followed by less than 20 years. Asphyxia and low to moderate apgar score was more common in prematures. The most common maternal risk factors were preclampsia and eclampsia, and urinary tract infection. Forty seven percent of LBW infants were hospitalized. The most common causes of admissions were sepsis and hyaline membrane disease (HMD). The mortality rate was 27.5% and the most important causes of mortality were HMD, asphyxia, and sepsis.

Conclusion : The incidence of LBW was within acceptable range. Birth weight is one of the most important factors in neonatal mortality and morbidity. The most important approach to the problem of LBW is prevention. This could be achieved not only by identification of it’s incidence and risk factors but also by improving the living standards and medical care of pregnant women