- Alpha-fetoprotein, the major fetal serum protein, is not essential for embryonic development but is required for female fertility. Gabant, P., Forrester, L., Nichols, J., Van Reeth, T., De Mees, C., Pajack, B., Watt, A., Smitz, J., Alexandre, H., Szpirer, C., Szpirer, J. Proc. Natl. Acad. Sci. U.S.A. (2002)