This population based study compares the rates of multiple births in the 1990s in four hospitals of different socioeconomic levels. It is well known that women from higher socioeconomic groups have easiest access to infertility therapies because of greater financial resources. The hospital of lower socioeconomic level presented multiple birth rates of approximately 8 per thousand during the decade, which may be considered as the natural one. The other three hospitals presented increased rates that were positively correlated to socioeconomic level. This increase occurred mainly due to dizygotic twins and to triplets and was as high as 4.8 per thousand in 1999. Maternal age was also positively correlated to socioeconomic level for singletons as well as for twins. However, during the decade the mean maternal age increased only in the two hospitals with better socioeconomic levels. Gestational order decreased as socioeconomic levels increased, mainly for twins and triplets. The percentage of singletons with low birthweight and very low birthweight decreased as socioeconomic level increased. However, twins presented with an equal distribution in the four hospitals, indicating that better socioeconomic level did not affect the incidence of low birthweight and very low birthweight among twins. Fetal death rate decreased as socioeconomic level increased but twin/singleton fetal death ratio is three times greater in the hospital of higher socioeconomic level suggesting that even in ideal conditions of medical and hospital facilities, the mortality of twins continues to be much higher than that of singletons.

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