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Unbound (free) estriol in blood
Last reviewed: 04.07.2025
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Estriol is the main estrogen synthesized by the placenta during pregnancy. Unbound estriol passes through the placenta and enters the pregnant woman's bloodstream, where it quickly turns into glucuronide and sulfate derivatives, which facilitates its excretion. The half-life of estriol in the blood of a pregnant woman is only 20-30 minutes. In this regard, its determination is a convenient and quick way to assess the current condition of the fetus. The concentration of estriol in the blood constantly increases throughout pregnancy and especially rapidly in its last third (28-40 weeks).
Free estriol concentration in pregnant women's blood serum
Gestational age, weeks |
Estriol, ng/ml |
28-30 |
3.2-12 |
30-32 |
3.6-14 |
32-34 |
4.6-17 |
34-36 |
5.1-22 |
36-38 |
7.2-29 |
38-40 |
7.8-37 |
A sudden decrease in estriol production results in a rapid fall in the concentration of its unbound fraction in the blood serum. Determination of unbound estriol has several advantages over determination of its total fraction in serum or urine, since it is independent of the presence of kidney or liverdisease and the use of various antibiotics. The concentration of unbound estriol more accurately reflects the probable outcome of pregnancy in patients with diabetes mellitus.
Estriol concentrations in body fluids are usually measured to determine the condition of the fetus, particularly in pregnant women with a high risk of premature birth or fetal death. Since the normal ranges of unbound estriol concentrations in serum are very wide, a single test is not enough. It is necessary to continuously monitor this parameter to establish the trend of its changes in each individual case. A persistently low concentration or a sudden and prolonged decrease during the last third of pregnancy usually indicates fetal pathology (including intrauterine death).
Diseases and conditions in which the concentration of free estriol in the blood serum changes
Increased concentration |
Decreased concentration |
A sharp increase in the likelihood of premature birth |
In case of pregnancy pathology (pronounced malformations of the central nervous system in the fetus, congenital heart defects, Down syndrome, fetal growth retardation, Rhesus conflict, fetal anemia, pyelonephritis, malnutrition, hemoglobinopathies, fetal adrenal hypoplasia, intrauterine fetal death). Uses of penicillin |