Urine estriol or total estrogens. Estriol is an estrogenic compound produced by the placenta from precursors derived from fetal adrenal cortex and fetal liver. Newly synthesized estriol is unconjugated; therefore, unconjugated estriol represents a product of the entire fetoplacental unit. The unconjugated estriol reaches maternal serum (where it has a half-life of about 20 minutes) and is taken to the maternal liver, where about 90% is conjugated with a glucuronide molecule. The conjugated form of estriol is excreted in maternal urine. A lesser amount of conjugated estriol is produced by the maternal liver from nonestriol estrogens synthesized by the placenta from maternal adrenal precursors. Serum estriol can be measured either as total estriol or as unconjugated estriol. It usually is measured as unconjugated estriol to exclude maternal contribution to the conjugated fraction. Urine estriol can be measured as total estriol or as total estrogens, since estriol normally constitutes 90%-95% of urine total estrogens.

Historically, urine total estrogen was the first test used, since total estrogen can be assayed by standard chemical techniques. However, urine glucose falsely increases results (which is a problem in diabetics, who form a large segment of the obstetrical high-risk group), and certain other substances such as urobilinogen also may interfere. In addition, urine total estrogen results are a little more variable than urine estriol patterns. Eventually, other biochemical procedures that were more specific for urine estriol (in some cases, however, the “estriol” being measured using biochemical methods is actually total estrogen) were devised. These procedures also have certain drawbacks, some of which are shared by the total estrogen methods. Both urine total estrogens and urine estriol have a significant degree of between-day variation in the same patient, which averages about 10%-15% but which can be as high as 50%. Both are dependent on renal excretion, and, therefore, on maternal renal function. Both have a maternal component as well as the fetal component. Urine total estrogen necessitates a 24-hour collection. The standard method for urine estriol also is a 24-hour specimen. There is substantial difficulty collecting accurate 24-hour specimens, especially in outpatients. Also, there is a 1- to 2-day time lag before results are available. Some have proposed a single voided specimen based on the estriol/creatinine ratio. However, there is controversy whether the single-voided specimen method (reported in terms of either estriol per gram creatinine or estriol/creatinine ratio) provides results as clinically accurate as the 24-hour specimen.

Estriol can be detected by immunoassay as early as the ninth week of gestation. Thereafter, estriol values slowly but steadily increase until the last trimester, when there is a more pronounced increase. Clinical use of estriol measurement is based on the fact that severe acute abnormality of the fetoplacental unit (i.e., a dead or dying placenta or fetus) is manifested either by failure of the estriol level to continue rising or by a sudden marked and sustained decrease in the estriol level. Urine specimens are usually obtained weekly in the earlier part of pregnancy, twice weekly in the last trimester, and daily for several days if a problem develops.

In general, urine estriol or estrogen excretion correlates reasonably well with fetal health. However, there are important exceptions. Only severe fetal or placental distress produces urine estrogen decrease of sufficient magnitude, sufficient duration, and sufficiently often to be reliable (i.e., mild disorder may not be detected). There is sufficient daily variation in excretion so that only a very substantial and sustained decrease in excretion, such as 40%-50% of the mean value of several previous results, is considered reliable. Some consider an estriol value less than 4 mg/24 hours (after 32 weeks’ gestation) strongly suggestive of fetal distress and a value more than 12 mg/24 hours as indicative of fetal well-being. Maternal hypertension, preeclampsia, severe anemia, and impaired renal function can decrease urine estrogen or estriol excretion considerably. Decrease may also occur to variable degree in variable numbers of fetuses with severe congenital anomalies. Certain drugs such as ampicillin and cortisol may affect urine estriol or estrogen values by effects on production, and other substances such as mandelamine or glucose can alter results from biochemical interference with some test methods. Some investigators have reported a decrease shortly before delivery in a substantial minority of normal patients. Maternal Rh-immune disease may produce a false increase in urine estriol levels. Continued bed rest has been reported to increase estriol excretion values an average of 20% over levels from ambulatory persons, with this increase occurring in about 90% of patients in the third trimester.

The literature contains widely differing opinions regarding clinical usefulness of estrogen excretion assay in pregnancy. In general, investigators have found that urinary estrogen or estriol levels are decreased in about 60%-70% of cases in which fetal distress occurs (literature range, 33%-80%). The more severe the fetal or placental disorder, the more likely that urine estrogen or estriol levels will be low. The percentage of falsely low values is also said to be substantial, but numerical data are not as readily available.

Serum unconjugated estriol. Plasma or serum unconjugated estriol, measured by immunoassay, has been used as a replacement for urine hormone excretion. Advantages include ease of specimen collection (avoidance of 24-hour urine collection problems), increased specificity for fetoplacental dysfunction (no maternal hormone contribution), no 24-hour wait for a specimen, closer observation of fetoplacental health (due to the short unconjugated estriol half-life), little technical interference by substances such as glucose, and less dependence on maternal kidney function. Drawbacks include the majority of those drawbacks previously described for urine hormone excretion (effect of bed rest, hypertension, and other conditions, and medications affecting estrogen production). Also similar to urine excretion, there is substantial between-day variation, averaging about 15% (reported maximum variation up to 49%). However, there are also considerable within-day fluctuations, which average about 15% (with maximum variation reported as high as 51%). Thus, a single value is even more difficult to interpret than a urine value. Some believe that 24-hour urine measurements may thus have some advantage, since within-day fluctuations are averaged out. Also similar to urine values, the current trend of interpretation is to require a sustained decrease of 40%-50% from the average of several previous serum values to be considered a significant abnormality. The serum specimens should preferably be drawn at the same time of day in the same patient position and assayed by the same laboratory. Although frequency of sampling is not uniform among investigators, many obtain one or two specimens per week during the earlier part of pregnancy and one per day if there is clinical suggestion of abnormality or one serum value becomes significantly decreased. Although there is some disagreement, the majority of investigators indicate that serum unconjugated estriol has a little better correlation with clinical results than does urine hormone excretion.

Because of the problems associated with collection of urine or serum estriol specimens and interpretation of the values, as well as the disturbing number of false positive and false negative test results, many clinicians depend more on other procedures (e.g., the nonstress test, which correlates the rate of fetal heartbeat to fetal movement) than on estrogen values to monitor fetal well-being.

Placental lactogen. HPL is a hormone produced only by the placenta, with metabolic activity similar in some degree to that of prolactin and GH. Values correlate roughly with the weight of the placenta and rise steadily in maternal serum during the first and second trimesters before entering a relative plateau in the third. Serum levels of hPL are higher than those attained by any other peptide hormone. Serum half-life is about 30 minutes. Although hPL cross-reacts with GH in most radioimmunoassay (RIA) systems, the high level of hPL relative to GH at the time of pregnancy when hPL levels are measured prevents clinical problems with GH interference. Serum hPL has been evaluated by many investigators as a test of placental function in the third trimester. Its short half-life is thought to make it a more sensitive indicator of placental failure than measurements of other hormones, especially urine measurements. Since hPL levels normally can fluctuate somewhat, serial measurements are more accurate than a single determination. Estriol, which reflects combined fetal and placental function, still seems to be used more than hPL.