Medical Genetics MSAFP testing is based on the fact that ds fetuses tend to be a little smaller on average, have smaller placentas, and thus secrete less msafp and other materials, which are determined in the serum of the pregnant woman. Factors that affect this test include gestational age, maternal weight, diabetes and ethnicityMedical Genetics MSAFP testing is based on the fact that DS fetuses tend to be a little smaller on average, have smaller placentas, and thus secrete less MSAFP and other materials, which are determined in the serum of the pregnant woman. Factors that affect this test include gestational age, maternal weight, diabetes and ethnicity