Toxoplasmosis in the foetus can be best confirmed by detecting which of the following serologic test?

Congenital toxoplasmosis can be diagnosed by the presence of a positive IgM antibody titer in the fetus after the first week of life. An increase in IgM beyond the first week of life is indicative of acute infection. If the mother gets infected more than 6 months before conception there is reduced risk of transmission of infection to the fetus. But the likelihood of transplacental infection increases as the interval between infection and conception decreases. The incidence of transplacental infection is the lowest (15%) in the first trimester and highest in the third trimester (65%). Only 20% of women infected with T gondii shows signs of infection. Infected infants who are normal at bih may have a higher incidence of learning disabilities and chronic neurologic sequelae than uninfected children. Ref: Harrison's Principles of Internal Medicine, 18e chapter 214.