A function for estriol during human pregnancy—a hypothesis

It has been hypothesized that large amounts of estriol (E 3) are produced during human pregnancy to ensure a quiescent uterus during prelabour pregnancy by combining with most of the myometrial nuclear receptors, leaving an inadequate number for a stimulatory estradiol (E 2) concentration. It is further hypothesized that the amount of E 3 formed is controlled by the amount of E 2 present. During labour this control is lost, which together with an increased E 2 (plus or minus a simultaneous drop in E 3) production permits labour. Two of the seven pieces of evidence offered in support of this hypothesis were carried out in the author's laboratory. These are: 1. Urinary assays by two methods, one for total estrogens and one for the fractionated classical estrogens revealed that while the ratio ▪ varies from patient to patient, for any one patient it remains markedly constant, especially during the second half of pregnancy. 2. Pieces of myometrium removed at cesarean delivery and assayed for their nuclear estrogen content revealed an E 3/E 2 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 labour had started. The relationship between these two pieces of evidence and five from the literature with the hypothesis are discussed.