Estriol (E3) is the major estrogen formed by the fetoplacental unit during pregnancy. Unconjugated E3 passes through the placenta into the maternal circulation, where it is rapidly converted into glucuronide and sulfate derivatives to facilitate its excretion. The half-life of estriol in the maternal bloodstream is only 20–30 minutes. Its measurement, therefore offers a convenient and quick evaluation of current fetal status. Plasma estriol levels increase steadily throughout pregnancy and most rapidly during the third trimester (28–40 weeks). A sudden decrease in fetoplacental E3 production will result in a rapid fall in unconjugated E3 in the maternal serum. There are several potential advantages to measuring unconjugated E3 rather than total serum or urinary E3. Unconjugated estriol levels are free from effects related to maternal renal or hepatic disease, and are not altered by the administration of certain antibiotics. Unconjugated E3 more accurately reflects fetal outcome in diabetic pregnancies – and since no hydrolysis of unconjugated E3 is required, a more rapid turnaround for the test result is possible.
- References to Estriol (E3)