A barrier which prevents the blood of a fetus and that of the mother from mixing, but allows water, oxygen and hormones to pass from mother to fetus.
The selective ability of the placental membranes to limit the exchange of substances between the maternal and fetal circulations. Although water, oxygen and other gases, drugs, needed nutrients (e.g., glucose and amino acids), maternal antibodies, and viruses cross the barrier unimpaired, large molecules, red blood cells, bacteria, and protozoa cross it only through breaks in placental integrity.