What is the largest antibody produced by the fetus?

The largest antibody produced by the fetus is Immunoglobulin M (IgM). It is the first antibody to be produced in response to an infection and plays a crucial role in the fetal immune response. IgM is typically produced by the fetus around the 20th week of gestation and is important for detecting and neutralizing pathogens. Unlike other antibody classes, IgM is pentameric in structure, allowing it to effectively bind to multiple antigens.