Prematurity, low birth weight, increased risk of infant death and long-term risk for morbidity associated with multiple gestations

The causes of temporal variation in the twinning rate are not always clear. As a consequence, the study of this variation has attracted the attention of several authors who were also interested in the correlation between twinning rate and human fertility. Prematurity, low birth weight, increased risk of infant death and long-term risk for morbidity associated with multiple gestations have been another source of motivation for epidemiological studies on twin births. In urban populations of countries where assisted fertilization is not widespread, a decline in dizygotic twinning has been observed that was positively correlated with parity, independent of age. With regard to the monozygotic twinning rate, most populations have shown stability of its incidence, but a few authors have observed a slight increase in monozygotic twin births. The rise in monozygotic twinning might be explained as a consequence of residual effects of long term use of oral contraceptives. Such effects include depression in tubal motility and changes in the endometrial mucosa and oviduct epithelium, which are considered as factors favoring the birth of monozygotic twins. Thus, in animal models, delayed implantation of the embryo is associated with polyembryony in armadillos, while delayed ovulation, frequently occurring in the first cycle after withdrawal of oral contraceptives, induces monozygotic twinning in rabbits. In Brazil, the hormone treatment on women to induce ovulation, whether followed by in vitro fertilization or not, is mostly performed in private clinics. Therefore, it is a reasonable supposition that women with a high standard of life attended at private hospitals would include a high proportion of those who have had assisted fertilization, which is associated with multiple births. Moreover, since women with high socioeconomic and cultural levels postpone procreation, their mean reproductive age tends to be higher than that observed in the general population. Thus, another cause influencing the increase in multiple births in these hospitals is maternal age, which is correlated with twinning, due to an increase of dizygotic pairs. This paper had the objective of estimating the frequency of multiple births in a first-class tertiary private hospital in Brazil.