The earliest immunoglobulin to be synthesized by the fetus is:

IgM is the earliest immunoglobulin to be synthesized by the fetus, beginning at about 20 weeks of age IgM holds special significance as it does not cross the placenta. It's presence in the fetus or neonate therefore indicates intrauterine infection and its detection is useful in the diagnosis of congenital infections. IgM antibodies are relatively sho lived disappearing earlier than IgG. Its demonstration in the serum therefore indicates recent infection. Ref: Textbook of Microbiology By Ananthanarayan and Panicker, 6th Edition, Page 83 ; Immunology: A Sho Course By Richard Coico, Geoffrey Sunshine, 2009, Page 53