Marked rises in both unconjugated and sulphoconjugated oestrone, oestradiol-17β and oestriol were observed in human foetal plasma between mid-gestation and term. Significant arterio-venous differences for the individual oestrogens in the unconjugated form were found in the umbilical cord plasma. No consistent arterio-venous differences were found for the oestrogen sulphates. This indicates that all three oestrogens are secreted from the placenta into the foetal circulation in the unconjugated form. Mean unconjugated oestrogen (oestrone + oestradiol-17β + oestriol) levels rose from 22·7 ng/ml at 17–20 weeks of gestation to 108·9 ng/ml at term in umbilical venous plasma and from 4·3 ng/ml to 23·3 ng/ml in umbilical arterial plasma. This represents a secretion rate of approximately 30 mg oestrogen/day into the umbilical vein at term. Mean oestrogen sulphate levels rose from 128 ng/ml to 313 ng/ml in the cord plasma during the same period. Of the three oestrogens measured, oestriol was quantitatively the major oestrogen in foetal plasma. It consistently represented about 78% of the unconjugated fraction and 95% of the sulphate fraction at all stages of gestation.
The method of delivery did not have a significant effect on the oestrogen levels in uncomplicated pregnancies. Similar oestrogen levels were found in foetal heart blood after either hysterotomy or spontaneous abortion at 16–20 weeks of gestation, and no significant differences were found for oestrogen levels in cord plasma after elective Caesarean section at 38–39 weeks when compared with oestrogen levels after normal delivery at term.
A significant rise in foetal unconjugated oestrogens at a time when foetal corticosteroids are increasing may be of physiological importance for foetal maturation in women.
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