* The placenta nourishes the fetus by establishing a dialysis pattern for exchange between the maternal and fetal circulations.

* The placental barrier is very thin and freely permeable to glucose and amino acids.

* The PO2 of the blood of the umbilical vein is the same as that in the maternal arterial blood.

* The estrogens secreted by the placenta tend to inhibit uterine excitability during pregnancy.