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Lecithin-to-sphingomyelin ratio (L/S ratio) in amniotic fluid

Published: August 9, 2026Updated: August 16, 2026

This test measures the balance of lecithin and sphingomyelin in your amniotic fluid. It helps assess fetal lung maturity, which is important before preterm birth.

Quick summary

["The L/S ratio helps determine if your baby’s lungs are ready for breathing outside the womb."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
27233-6
Short code
Lecithin Amn-aCnc
Synonyms
Lecithin Amn-aCnc
Category
chemistry
Sample
fluid
Container
Sterile tube
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Specimen volume
2-5 mL
Stability
Room temperature: 2h, Refrigerated: 24h
Hemolysis max
300
TAT
4–24 hours
Order
routine

What Is the L/S Ratio Test Used For?

Doctors use this test to check if your baby’s lungs are mature enough to function properly after birth. A higher lecithin-to-sphingomyelin ratio suggests healthy lung development, which is critical for avoiding respiratory distress syndrome in preterm infants.

Why Is This Test Important During Pregnancy?

If your due date is early or you experience preterm labor, this test can guide decisions about whether to delay delivery or prepare for early intervention. It helps ensure your baby’s lungs are ready to support breathing independently.

How Is the Test Performed?

A small sample of amniotic fluid is collected through an amniocentesis procedure. The fluid is then analyzed in a lab to measure the relative amounts of lecithin and sphingomyelin, which are key components of fetal lung surfactant.

What Do the Results Mean?

Results are typically reported as a ratio (e.g., 2:1 or higher). A ratio of 2:1 or greater usually indicates mature lungs, while lower ratios suggest immature lungs. Your healthcare provider will interpret the results in the context of your pregnancy timeline.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance: The L/S Ratio Plays a Critical Role in Managing Preterm Birth Risks

Short name

Amniotic fluid L/S ratio

Frequently asked questions

What happens if my L/S ratio is low?
A low L/S ratio may indicate that your baby’s lungs are not fully developed. Your doctor might recommend further monitoring, medical interventions (like corticosteroids to speed lung maturity), or planning for early delivery if preterm labor is confirmed.
How often is this test repeated if needed?
If your L/S ratio is borderline or low, your doctor may repeat the test or administer corticosteroids (medication) to help your baby’s lungs mature faster. Follow-up testing is usually done 24–48 hours later.
Can this test be done at home?
No, this test requires a medical procedure (amniocentesis) and is only performed in a hospital or clinic setting by a healthcare professional.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy