Toxoplasmosis in the foetus can be best confirmed by-

IgM can be detected within the first 2 weeks of infection using the ISAGA or ELISA methods. .Women who are positive for IgG and negative for IgM are defined as immune and no fuher follow-up is necessary, because their fetuses are not at risk of congenital toxoplasmosis Ref Harrison20th edition pg 1312