Phosphatidyl Glycerol

The ability of the fetus to survive outside of the uterus is greatly dependent on proper pulmonary function. Pulmonary surfactant is needed to reduce the surface tension of the air-liquid interface of the alveolar lining, so that alveoli don’t collapse upon expiration.  Infants born with a deficiency of pulmonary surfactant are at increased risk of developing respiratory distress syndrome (RDS). The incidence of RDS is dependent on gestational age, with more RDS occurring in younger fetuses. For instance, the risk of RDS is 0% at 40 weeks, 2% at 36 weeks, and 8 to 23% at 34 weeks, depending on birth weight. RDS can occur at term, especially with Rh isoimmunization and maternal diabetes.

Generally, fetal lung maturity testing was not necessary when the gestational age is >39 weeks because fetal lungs are usually mature. Unfortunately, complications such as diabetes and Rh isoimmunization retarded fetal lung development. Women with these complications may require testing. Testing was also not usually necessary when the gestational age was <30 weeks, because most fetuses are expected to have immature lungs. However, a number of disorders accelerate pulmonary maturity including maternal hypertension, preeclampsia, HELLP syndrome, premature rupture of the membranes, intrauterine growth restriction, maternal smoking and drug use, and maternal hemoglobinopathies. Women with one of these complications and preterm labor or premature rupture of the membranes may have required testing for fetal lung maturity (FLM).

Surfactant is a complex mixture of lipids, proteins, and carbohydrates.  Most of the lipids are phospholipids including lecithin (L), sphingomyelin (S), disaturated lecithin, phosphatidyl inositol (PI), and phosphatidyl glycerol (PG). All of these phospholipids are manufactured by the fetal lung, possess surface tension activity, and are secreted into the amniotic fluid.  Lecithin is present in the lung in the highest concentration, but must be stabilized by PI and/or PG to have maximal surface tension reducing activity. The relative concentrations of these phospholipids determine the degree of fetal lung maturity. 

PG usually becomes detectable in amniotic fluid at about 35 or 36 weeks gestation. Because PG appears later in gestation, it is a good indicator of maturity and usually indicates there is minimal risk for neonatal respiratory distress. PG is detected using the AmnioStat-FLM test. The positive predictive value is greater than 95%. 

For a more comprehensive discussion, see “Fetal Lung Maturity Profile.”

References

Grenache DG et al. Clinical and laboratory trends in fetal lung maturity testing, Clinica Chimica Acta 2010;411:1746-1749.

ACOG Committee Opinion No. 765: Avoidance of Nonmedically Indicated Early-Term Deliveries and Associated Neonatal Morbidities. Obstetrics and Gynecology 133(2) (2019) e156-e163.

ACOG Committee Opinion No. 764: Medically Indicated Late-Preterm and Early-Term Deliveries. Obstetrics and Gynecology 133(2) (2019) e151-e155.