A FUNCTION FOR ESTRIOL DURING HUMAN-PREGNANCY - A HYPOTHESIS

It was hypothesized that large amounts of estriol (E3) are produced during human pregnancy to ensure a quiescent uterus during prelabor pregnancy by combining with most of the myometrial nuclear receptors, leaving an inadequate number for a stimulatory estradiol (E2) concentration. It is further hypothesized that the amount of E3 formed is controlled by the amount of E2 present. During labor this control is lost, which together with an increased E2 (plus or minus a simultaneous drop in E3) production permits labor. Two of the 7 pieces of evidence offered in support of this hypothesis were examined. These are: Urinary assays by 2 methods, 1 for total estrogens and 1 for the fractionated classical estrogens revealed that while the ratio (E3/(Total Estrogen-E3))=(E3/(E2 .tautm. Estrone (E1))) varies from patient to patient, for any 1 patient it remains markedly constant, especially during the 2nd half of pregnancy. Pieces of myometrium removed at cesarean delivery and assayed for their nuclear estrogen content revealed an E3/E2 ratio of 1.7 when the cesarean was an elective one (and therefore with a quiescent uterus) but reduced to 0.65 when the cesarean was performed after labor had started. The relationship between these 2 pieces of evidence and 5 from the literature with the hypothesis were discussed.