In this study, the frequency of twin births was determined at the Federal University of Santa Maria Hospital (HUSM), during the period 1985-1995, when 253 double pregnancies (1.58%) and 6 triple pregnancies (0.037%) occurred in a 16006 births. Using the Weinber method, corrected by James, a frequency of 44.7% of dizygotic pairs and therefore of 55.25% of monozygotic pairs was established. The sex ratio analysis showed 85.82 male children per 100 female among the twins, a result that differed significantly in single births, where 103.89 males per 100 females occurred. These results were compared with those found in other populations.
