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Lecithin/Sphingomyelin Ratio (L/S Ratio) in Amniotic Fluid

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

This test measures the ratio of two fats found in amniotic fluid to help assess fetal lung maturity before birth.

Quick summary

["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
2557-7
Short code
Lecithin/Sphingomyelin Amn
Synonyms
Lecithin/Sphingomyelin Amn
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

Why This Test Is Done

Your healthcare provider may recommend this test if you are at risk for preterm labor or if your due date is approaching. It helps ensure your baby’s lungs are developed enough to breathe on their own after birth.

How the Test Works

A small amount of amniotic fluid is collected through a needle inserted into the uterus. The fluid is then analyzed to measure the ratio of lecithin (a fat that helps with lung function) to sphingomyelin (another fat).

What to Expect Before the Test

You’ll likely be monitored closely during the procedure, which may include lying on your back with your legs raised. You may feel mild cramping, but it’s usually not painful.

What to Expect During the Test

The test itself takes a few minutes. You may need to stay in the hospital for observation afterward, depending on your provider’s protocol.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

A higher L/S ratio (typically ≥2:1) suggests mature lungs and reduces the risk of respiratory distress syndrome in newborns. A lower ratio may indicate immature lungs, prompting further evaluation or interventions like corticosteroids to help develop the lungs faster.

Short name

Amniotic Fluid L/S Ratio

Frequently asked questions

What does an abnormal result mean?
An abnormal result may mean your baby’s lungs aren’t fully developed. Your healthcare team will discuss next steps, such as additional testing or treatments to support lung maturity.
How soon can I expect results?
Results are usually available within a few hours to a day, depending on the lab’s processing time. Your provider will share the findings with you promptly.
Is there anything I should avoid before the test?
No special preparations are needed, but inform your provider if you have any infections or conditions that might affect the fluid sample.
  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