Fetal Lung Maturity - Neonatal Respiratory Distress Syndrome

Key Points

Fetal lung maturity (FLM) tests have historically been performed to predict whether a fetus’s lungs are developed enough for delivery. However, FLM testing has limited value in light of the most recent ACOG guidelines (2013), which advise against delivery <39 weeks unless medically mandated due to potential serious morbidity when compared to those delivered ≥39 weeks in spite of mature FLM tests.

FLM testing may have value in the following clinical situations:

  • Premature rupture of membranes (≥32 weeks) – if FLM test is mature, delivery is likely safer than “wait and see” approach
  • Assessment of need for NICU – possible only if early delivery has medical mandate and time allows for FLM testing
  • Other selected late preterm and early preterm pregnancy issues where FLM may guide management of at-risk pregnancy

Comparison of FLM Laboratory Testing Options (all testing requires amniotic fluid)

Lamellar body count (LBC)

Phosphatidylglycerol (PG)

Lecithin-sphingomyelin ratio (L/S)

  • Initial FLM of choice
  • Rapid, sensitive
  • New data indicates that one can estimate risk of respiratory distress syndrome (RDS) as a function of gestational age and LBC
  • Unclear whether *ACOG cascade should be followed if LBC is immature
  • Not useful unless gestational age ≥35 weeks
  • Limited availability
  • Sensitive
  • Unclear whether *ACOG cascade should be followed if PG is immature
  • Main role is in adjudication of immature LBC or PG
  • Last test of choice
    • Labor intensive, imprecise
    • Limited availability
    • Results take >24 hrs unless performed at a local laboratory

* Refer to Diagnosis section for ACOG cascade

Note: in general, mature results suggest RDS unlikely


ACOG Practice Bulletin No. 107: Induction of Labor. Obstet Gynecol. 2009; 114 (2) :386-397.

ACOG Practice Bulletin No. 97: Fetal lung maturity. Obstet Gynecol. 2008; 112 (3) :717-726.


Indications for Testing

  • Ruptured membranes ≥32 weeks – if fetal lung maturity (FLM) test is mature, delivery is likely safer than “wait and see” approach
  • Assessment of need for NICU – possible only if early delivery has medical mandate and time allows for FLM testing

Laboratory Testing

  • “Cascade” testing advocated by ACOG
    • Fetal Lung Maturity Test Cascade (ACOG)

  • Available FLM tests
    • Fetal Lung Maturity Tests

      Fetal Lung Maturity Tests


      Lamellar body count (LBC)

      Phosphatidyl-glycerol (PG)

      ratio (L/S)


      cell count

      Thin layer chromatography

      Thin layer

      Immaturity cutoff

      None; there is no LBC result below which fetal lung immaturity can be identified with high certainty



      Maturity cutoff




      Negative (mature) predictive value




      Positive (immature) predictive value




      Effect of blood        

      Decreases count due to trapping of lamellar bodies in clot matrix


      Mature results will be decreased; immature results will be increased

      Mature results are still interpretable

      Effect of meconium

      Increases count


      Makes L/S ratio result unreliable

Clinical Background

Prematurity is associated with numerous complications, including neonatal respiratory distress syndrome (RDS), a cause of infant morbidity and mortality.


  • Incidence – 20/100,000 infant deaths due to RDS
  • Age – more common the younger the gestational age
  • Sex – M>F (minimal)


  • Pulmonary surfactants are synthesized by type II pneumocytes and packaged into storage granules called lamellar bodies; these function to decrease alveolar surface tension
    • Lecithin – detected at week 28; surges at week 36
    • Phosphatidylinositol – detected at week 28; peaks at week 35
    • Sphingomyelin – detected at week 28
    • Phosphatidylglycerol – detected at week 36 with increases until delivery
  • RDS is caused by insufficient concentrations of pulmonary surfactants, resulting in collapsed alveoli (alveoli are perfused but hypoventilated)
    • Leads to hypoxia, hypercapnia, and respiratory acidosis
    • Conditions cause vasoconstriction of pulmonary arteries and decreased pulmonary blood flow
      • Pulmonary vasoconstriction causes epithelial cell damage, allowing plasma to leak into alveoli
        • Fibrin accumulation and necrotic cells create a hyaline membrane (RDS previously called hyaline membrane disease)
  • Nearly always associated with preterm birth
    • Risk of RDS is inversely related to gestational age at birth
      • >60% at <30 weeks
      • 20% at 34 weeks
      • <5% at >36 weeks
    • Measurement of fetal lung maturity through biochemical testing of amniotic fluid helps predict risk of RDS

Clinical Presentation

  • Respiratory distress that occurs within the first few hours of life – almost exclusively in preterm infants
  • Hypoxia, hypercapnia, and acidosis ensue with respiratory failure in many neonates

Indications for Laboratory Testing

  • Tests generally appear in the order most useful for common clinical situations
  • Click on number for test-specific information in the ARUP Laboratory Test Directory
Test Name and Number Recommended Use Limitations Follow Up
Lamellar Body Counts 0080940
Method: Quantitative Automated Cell Count

Preferred fetal lung maturity test

Rapid, sensitive

Do not apply reference values for test to other instruments without performing comparison studies  Unclear whether ACOG cascade should be followed if LBC is immature
Lecithin-Sphingomyelin Ratio 0080200
Method: Quantitative Chromatography

Preferred fetal lung maturity test is lamellar body counts; lecithin-sphigomyelin ratio may be useful adjucating immature LBC or PG

Labor intensive, imprecise

Limited availability

Results take >24 hrs unless performed at a local laboratory

Additional Tests Available
Click the plus sign to expand the table of additional tests.
Test Name and NumberComments
Fetal Fibronectin 0082024
Method: Semi-Quantitative Immunoassay/Immunosorbent

Assess risk of preterm delivery