Fetal Lung Maturity, Indicators
phospholipid component of pulmonary surfactant
absent in the amniotic fluid early in gestation.
appearance in the amniotic fluid at approximately 35 weeks' gestation coincides with fetal lung maturity and resultant production of surfactant-rich fetal lung fluid.
Thus, the testing of amniotic fluid for PG is useful as a marker for predicting fetal lung maturity.
Researchers have found that fewer than 1% of newborns who had PG in the amniotic fluid developed respiratory distress compared with 83% of newborns whose amniotic fluid was negative for PG.
produced by the fetal liver and embryonic yolk sac
the major fetal/embryonic serum protein early in gestation.
AFP is present in the amniotic fluid initially through diffusion across immature fetal skin and later through fetal urination.
AFP concentration in the amniotic fluid peaks in the midtrimester of pregnancy.
Amniotic AFP concentrations may be elevated in pregnancies complicated by fetal anomalies such as neural tube defect and abdominal wall defect.
Conversely, amniotic AFP concentrations may be depressed in pregnancies complicated by Down syndrome.
measurement of AFP in the amniotic fluid is more useful as a marker of fetal abnormalities than as an indicator of fetal lung maturity.
a membrane lipid
nonspecific component of amniotic fluid that is unrelated to fetal lung maturation.
The sphingomyelin content of the amniotic fluid tends to decrease from about 32 weeks' gestation to term.
In contrast, the lecithin content, a large part of which is derived from the fetal lung, increases during the same period.
Thus, the usefulness of the lecithin/sphingomyelin ratio as a marker of fetal lung maturity is based more on the rising content of lecithin in the amniotic fluid than on the steady or declining content of sphingomyelin.
Surfactant protein A (SP-A)
35-kd surfactant apoprotein in the amniotic fluid
may be useful as a nonphospholipid marker of fetal lung maturity.
Studies to date, however, have shown conflicting results with regard to sensitivity, specificity, and ease of performance of the test.
Further research is needed before SP-A assays gain widespread clinical acceptance.
Although thyroid hormones are important in fetal lung
maturation, measurement of thyroxine in the amniotic fluid is of limited or no
value as a marker of fetal lung maturity. Because the placenta is largely
impermeable to the thyroid hormones, the concentration of thyroid hormones in
the amniotic fluid is limited.
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