CD40LG and infection: However, interpretation of serologic testing can be challenging for the following reasons: positive newborn IgG can reflect past or current infection in the mother since it crosses the placenta; small amounts of maternal IgM can cross the placenta, resulting in low IgM levels in uninfected newborns if performed soon after birth; otherwise, if infection happens late in pregnancy, cases of false-negative IgM may occur, as IgM appears within 1-2 weeks of exposure; and maternal therapy can alter the serological profile of the newborn since it can delay IgG production [12].