Toxoplasmosis acquired in an immunocompetent individual will usually be asymptomatic. However, the problem arises in the case of a primary infection in a pregnant woman, which may cause congenital infection of the fetus. Through maternal-fetal contamination, this infection poses two main risks to the fetus and the child, one neurological (hydrocephalus and intracranial calcifications) and the other ophthalmic (chorioretinitis that can affect the visual prognosis), which makes preventive actions necessary, through compulsory monthly serological surveillance of women who are not immune to toxoplasmosis. The biologist is often confronted with difficulties in interpreting the results, particularly during the monitoring of pregnant women. However, this is an essential step in the prevention of congenital toxoplasmosis because the results of the serological analysis are the basis for subsequent prenatal and postnatal treatment serodiagnosis is based on the detection of specific IgG and IgM. Although serological techniques have improved in recent years, problems of interpretation of serology persist, particularly in the presence of IgM antibodies. Results should be interpreted with caution. The mistake not to be made is to conclude immediately that the patient is primitively infected on the basis of the presence of IgM or IgG antibodies associated with IgM antibodies alone. The knowledge of the evolution of antibodies during the infection and the practice of complementary tests in particular the measurement of the avidity of antitoxoplasmic IgG allow in the majority of the cases a correct interpretation of the results and a dating of the infection.