Work presented as a Master's Thesis at the Federal University of Goiás, 2009. It was supervised by Prof. Dr. Marize Martins Avelino and aimed to study the value of the IgG avidity test, a serological test used to confirm or not chronic toxoplasmosis disease and other transmissible diseases, as a reliable marker of vertical transmission and fetal infection rates in toxoplasmosis during pregnancy.