Abstract
Human chorionic somatomammotropin concentration in serum (S-HCS) during the latter half of pregnancy was measured by radioimmunoassay and correlated to the placental weight, in two groups of normal, healthy, pregnant women. In one group, 228 samples from 47 women were examined, which gives a longitudinal series. In the other group, single samples from each of 346 pregnant women were examined, which gives a cross-sectional series. Both groups were randomized on the basis of a prospective selection. The mean values of S-HCS in each week of gestation were almost identical in the two groups, showing a steady increase until 37-38 weeks and a subsequent decrease. In the longitudinal series there was a positive correlation between the S-HCS and placental weight after 37 weeks' gestation, but not before that time. Before 37 weeks gestation the ratio S-HCS/placental weight was significantly higher with small placentae than with large placentae. This difference between small and large placentae disappeared after 37 weeks. These results point to the existence of some regulatory mechanism tending to keep the S-HCS concentration within certain limits, independent of placental weight. This mechanism appears to be lost after 37 weeks of gestation when the S-HCS concentration starts to correlate with placental weight.