Hormone Ontogeny in the Ovine Fetus. II. Ovine Chorionic Somatomammotropin in Mid and Late Gestation in the Fetal and Maternal Circulations*

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Abstract
The plasma concentration of ovine chorionic somatomammotropin (oCS) was measured by homologous RIA in samples obtained from 56 chronically catheterized ovine fetuses, aged 66 days of gestation to term (147 days), and from their mothers. oCS concentrations from either ewe or fetus within 5 days of surgery were elevated compared to samples taken at least 6 days after surgery. In fetuses at least 6 days postoperative, a negative correlation (P < 0.001) was observed between fetal oCS concentration and gestational age. Between 81–100 days of gestation, the mean fetal oCS concentration was 80.0 ± 2.0 ng/ ml, and by 131–140 days of gestation, it had fallen to 37.7 ± 4.1 ng/ml. The mean maternal oCS concentration between 80–100 days of gestation was 54.5 ± 23.4 ng/ml and rose to a maximal concentration of 433.5 ± 93 ng/ml at 121–130 days. The ratio of fetal to maternal oCS concentration was 1.2 ± 0.4 in pregnancies less than 100 days of gestation and fell to 0.14 ± 0.04 at term. Both fetal and maternal oCS concentrations between 111–130 days of gestation were significantly higher in twin as compared to singleton pregnancies. These results demonstrate that the relative concentration of oCS in fetal and maternal circulations changes as gestation progresses. Fetal acidemia in late gestation was associated with marked increases in fetal oCS concentrations.