Fetal Lung Maturity, Indicators

Phosphatidylglycerol (PG)

• 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.

AFP (Alpha-fetoprotein)

• 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.

Sphingomyelin

• 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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