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Free estriol in serum
Last reviewed: 04.07.2025
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Estriol is the main steroid hormone synthesized by the placenta. In the first stage of synthesis, which occurs in the embryo, cholesterol, formed de novo or coming from the blood of the pregnant woman, is converted to pregnenolone, which is sulfated by the adrenal cortex of the fetus into DHEAS, then converted in the liver of the fetus into α-hydroxy-DHEAS, and then into estriol in the placenta. As a rule, the content of estriol in the blood of a pregnant woman correlates with the activity of the adrenal glands of the fetus. Estriol enters the bloodstream of the pregnant woman, where the concentration of its unconjugated form can be determined.
In case of pathology (pronounced malformations of the central nervous system in the fetus, congenital heart defects, Down syndrome, fetal growth retardation, fetal adrenal hypoplasia, intrauterine fetal death), the concentration of free estriol in the pregnant woman’s blood serum decreases.
In Down and Edwards syndrome, the concentration of free estriol is usually 0.7 MoM.
Estriol concentration in blood serum in the dynamics of physiological pregnancy
Gestational age, weeks |
Median, nmol/l |
Reference values, nmol/l |
6-7 |
1,2 |
0.6-2.5 |
8-9 |
1.6 |
0.8-3.5 |
10-12 |
4 |
2.3-8.5 |
13-14 |
8 |
5.7-15 |
15-16 |
10 |
5.4-21 |
17-18 |
12 |
6.6-25 |
19-20 |
15 |
7.5-28 |
21-22 |
24 |
12-41 |
23-24 |
28 |
18.2-51 |
25-26 |
31 |
20-60 |
27-28 |
32 |
21-63.5 |
29-30 |
35 |
20-68 |
31-32 |
38 |
19.5-70 |
33-34 |
43 |
23-81 |
35-36 |
52 |
25-101 |
37-38 |
64 |
30-112 |
39-40 |
65 |
35-111 |
Values of median serum free estriol concentrations for screening of congenital malformations in the second trimester of pregnancy
Pregnancy period |
Medians for free estriol, nmol/L |
15 |
4.3 |
16 |
4.8 |
17 |
5.5 |
18 |
6.4 |
19 |
7.1 |
20 |
8.2 |
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