HCG is measured in the amniotic fluid, in the plasma of the umbilical vein and of the umbilical artery. The values are as 3:1.5:1. From the 9th week of gestation onwards appears a striking rise in the HCG concentration in the amniotic fluid reaching peak values around the 13th week. This peak is followed by a substantial decrease reaching relative constant and low values, which are sustained until the 40th week. The HCG reaches the amniotic fluid by transamnial or transchorial way. Only small amounts come from the foetus. The increased HCG concentration in the amniotic fluid and in the blood of the umbilical vein in severe erythroblastosis and in diabetes mellitus could be caused by an increased production of HCG in the placenta and/or by a decreased inactivation of HCG by means of the foetus.