To investigate the association of Yellow Fever Vaccination (YFV) during pregnancy with the presence of structural defect in exposed babies. An observed/expected frequencies study, before and after the vaccination campaign against YF was designed. 304 babies exposed to YFV during the prenatal period underwent dysmorphological examinations. The expected frequencies of malformations were obtained from a reference population of 10,691 births occurred in the period immediately prior to the vaccination campaign and born in the same region. These frequencies were evaluated using Poisson distribution model. The major malformation rate found in this study was 3.3% (CI 1.7-6.3%). Minor dysmorphisms, especially naevus, were significantly more frequent (P<0.001) than in the reference population. The data here presented provide no indication that immunization with YFV early in pregnancy increases the risk of major malformations. However, the association found between YFV during pregnancy and minor dysmorphisms, especially pigmented naevus, seems to be a bias of evaluation. We suggest, nevertheless, that a reproductive risk hypothesis regarding minor dysmorphisms should be considered in future studies involving YFV.12833-7