This study aimed to compare birthweight distribution of twins and singletons from three different socioeconomic level hospitals and evaluate the possible contribution of assisted reproduction technologies (ART). Data for twins and singletons were collected in the 1990s from hospitals A (370 twins, 370 singletons), B (306 twins, 306 singletons), and C (562 twins, 562 singletons). Only hospitals B and C have ART procedures. Gestational age was significantly lower in hospital C for singletons and twins. Birthweight for singletons was lower at hospital A (p < .005 for hospital B and p = .000 for hospital C); however, birthweight for twins was lower for hospitals A and C compared to hospital B (p = .000 for both comparisons). There were no differences between the mean birthweight for singletons and twins either to primigravidae or multigravidae in hospital A; nevertheless, for B and C the mean birthweight of twins was significantly lower in primigravidae than in multigravidae (p = .029 and p = .006, respectively). Considering twins up to 37-weeks of gestational age, hospital C showed the highest percentage of twin births (73.3%). These data suggest that the use of ART accounts for a disproportionate number of low birthweight and/or premature infants in primigravidae of higher socioeconomic level.