Fetal and Placental Viability Testing
Fetal and placental viability is most often assessed in the third trimester, when the rapidly-growing fetus places greatest demands on the placenta. These tests are particularly useful in high risk pregnancies (maternal diabetes, hypertension, and other disorders including substance abuse) and in the evaluation of intrauterine growth retardation. The most common assessment for fetal viability is the correlation between fetal heart rate and fetal movement (the nonstress test). In cases where more information is needed, measurement of estriol and/or placental lactogen in maternal serum, particularly in serial specimens, can be used to monitor fetal status.
Estriol
Estriol is produced by the placenta from precursors that are synthesized in the fetal adrenal and liver. Thus estriol synthesis reflects the metabolic status of both the fetus and the placenta. Free estriol in the maternal circulation is conjugated (e.g., with glucuronide) in the maternal liver, released back into the bloodstream and excreted in the urine. Although total estriol, other serum estrogens and urine estrogens have been evaluated as indicators of fetal status, unconjugated serum estriol is considered to be the most accurate measure because it is completely a product of the fetus and placenta.
DHEA-S: Dehydroxyepiandrosterone Sulfate
Serum estriol levels steadily increase throughout pregnancy. A leveling-off of this increase, or a sustained drop in serum estriol levels indicates impending fetal demise.
Placental lactogen (hPL)
hPL is produced only by the placenta and levels generally correlate with placental weight. Levels normally rise over the first two trimesters and flatten out a bit in the third. hPL is used as a measure of placental funtion in the third trimester and in post-dates pregnancies (pregancies that are past term); its short half-life (about 30 min) means that it can provide a rapid assessment of the status of the placenta. hPL levels vary to a degree over a day, however, and therefore a comparison of serial specimens for a sustained trend gives the most accurate results. Declining values indicate fetal jeopardy. Low hPL with high hCG suggests trophoblastic (molar) disease.
Last modified:
1/22/97; Author:
J. Harrison