Classically, Toxoplasma infection is associated with high levels of specific IgM antibody and a rise in specific IgG levels 1 to 3 weeks later. Atypical IgG seroconversion, without IgM detection or with transient IgM levels, has been described during serologic follow-up of seronegative pregnant women and raises difficulties in interpreting the results. To evaluate the frequency and the characteristics of these atypical cases of seroconversion, an investigation was conducted within the French National Reference Center for Toxoplasmosis, from which 26 cases collected from 12 laboratories belonging to the network were identified. The aim of this work was to retrospectively analyze the results of serologic testing, the treatments administered, and the results of prenatal and postnatal follow-up for these women. In each case, IgG antibodies were detected using both screening and confirmatory tests. IgM antibodies were not detected in 15 cases, and the levels were equivocal or low-positive in 11 cases. The IgG avidity results were low in 16 cases and high in one case. Most of the pregnant women (22/26) were treated with spiramycin from the time that IgG antibodies appeared until delivery. Amniotic fluid was analyzed for Toxoplasma gondii DNA by PCR in 11/26 cases, and the results were negative in all cases. Congenital toxoplasmosis was ruled out in 12/26 newborns. There was no abnormality observed at birth for 10 newborns and no information available for 4 newborns. In conclusion, when the interpretation of serological results is so difficult, it seems cautious to initiate treatment by spiramycin and to follow the pregnant women and their newborns.