Which test is used to assess fetal lung maturity by measuring the ratio of phospholipids?

• A. Contraction stress test.
• B. Internal fetal monitoring.
• C. Abdominal ultrasound.
• D. Lecithin-sphingomyelin ratio.

Answer: (D)

Lecithin-sphingomyelin ratio in amniotic fluid is used to assess fetal lung maturity. Surfactant production, which prevents alveolar collapse after birth, increases as gestation progresses. Lecithin (phosphatidylcholine) is a key surfactant component that rises with fetal age, while sphingomyelin remains relatively stable. By sampling amniotic fluid via amniocentesis and measuring these two phospholipids, labs calculate the L/S ratio. A ratio around 2:1 or higher generally indicates the lungs are mature enough to function after birth, though some settings may use slightly different thresholds. Additional markers like the presence of phosphatidylglycerol can reinforce the assessment of maturity.

The other tests listed don’t evaluate lung maturity through surfactant measures. A contraction stress test looks at fetal heart rate response to contractions to gauge well-being during labor, not lung readiness. Internal fetal monitoring and abdominal ultrasound assess fetal status or anatomy rather than surfactant levels in the lungs.