Amniotic fluid contains numerous biomolecules derived from fetus and mother, thus providing precious information on pregnancy. Here, we evaluated oxidative stress of human amniotic fluid and measured the concentration of catalytic Fe (II). Amniotic fluid samples were collected with consent from a total of 89 subjects in Nagoya University Hospital, under necessary medical interventions: normal pregnancy at term, normal pregnancy at the 2nd trimester, preterm delivery with maternal disorders but without fetal disorders, congenital diaphragmatic hernia, fetal growth restriction, pregnancy-induced hypertension, gestational diabetes mellitus, Down syndrome and trisomy 18. Catalytic Fe (II) and oxidative stress markers (8-hydroxy-2'-deoxyguanosine, 8-OHdG; dityrosine) were determined with RhoNox-1 and specific antibodies, respectively, using plate assays. Levels of 8-OHdG and dityrosine were higher in the 3rd trimester compared with the 2nd trimester in normal subjects, and the abnormal groups generally showed lower levels than the controls, thus suggesting that they represent fetal metabolic activities. In contrast, catalytic Fe (II) was higher in the 2nd trimester than the 3rd trimester in the normal subjects, and overall the abnormal groups showed higher levels than the controls, suggesting that high catalytic Fe (II) at late gestation reflects fetal pathologic alterations. Notably, products of H 2 O 2 and catalytic Fe (II) remained almost constant in amniotic fluid.