Maximum level of alpha fetoprotein is seen in:
Fetal serum
Placenta
Amniotic fluid
Maternal serum
Alpha fetoprotein is the most abundant protein in the fetal serum throughout fetal development.
It is transferred from fetus to amniotic fluid when fetus passes urine.
The concentration of AFP in amniotic fluid is approximately 100 folds less than in fetal serum, peaks at 13-14 weeks and then decreases in the second trimester (by 10% per week).
AFP reaches the maternal serum by diffusion across the amniotic membranes and via the placenta.
The level of AFP in maternal serum is less than fetal serum as is suggested by:

“Fetal serum contains AFP in a concentration 150 times that of maternal serum”.
COGDT 10/e, p 185
“Ordinarily high level of fetoprotein are found in developing fetus and low levels exist in maternal serum and amniotic fluid”.
Mannual of Laboratory and Diagnostic Test Fishback 7/e, p 995