FIVNAT registry collected 4,323 clinical pregnancies from 1987 to 1991, and allowed us to analyse the risk factors for multiple pregnancies. The multiple pregnancy rate is around 28%, 23% for twin pregnancies and 5% for triplet pregnancies. The number of transferred embryos is not the only factor to be significantly related with the risk of multiple pregnancy: male infertilities, cleavage rate over 50%, and women's age (at young ages multiple pregnancy rate are higher) are also significant. A multivariate logistic model was applied to analyse the relative part of these different factors. We also estimated the success rate and the multiple pregnancy rate which should have been obtained if the number of transferred embryos has been limited, in order to reduce the multiple pregnancy rate to 3%.