Skip to content

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

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

This test measures the ratio of lecithin to sphingomyelin in amniotic fluid to help assess fetal lung maturity.

Quick summary

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

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

Key facts at a glance

LOINC
14976-5
Short code
Lecithin/Sphingomyelin Amn-Rto
Synonyms
Lecithin/Sphingomyelin Amn-Rto
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 Is This Test Done?

Your healthcare provider may recommend this test if you’re at risk for preterm labor or if your due date is approaching. It helps identify whether your baby’s lungs are developed enough to breathe on their own outside the womb.

How Is the Test Performed?

A small amount of amniotic fluid is collected through an amniocentesis procedure. The fluid is then analyzed in a lab to measure the lecithin and sphingomyelin levels.

What Does the Result Mean?

A higher lecithin-to-sphingomyelin ratio (typically greater than 2:1) suggests healthy lung development. Lower ratios may indicate immature lungs, which could affect your baby’s breathing at birth.

What Are Possible Outcomes?

If the L/S ratio is low, your healthcare provider may recommend additional monitoring or a more immediate delivery plan. If the ratio is high, it may suggest readiness for birth.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

The L/S ratio is a key test in evaluating fetal lung maturity, particularly in high-risk pregnancies. It helps guide decisions about timing of delivery to minimize risks for both mother and baby, such as respiratory distress in newborns.

Short name

L/S Ratio Test

Frequently asked questions

What Does an L/S Ratio Test Involve?
The test involves a procedure called amniocentesis, where a small needle is used to collect amniotic fluid from your uterus. This fluid is then sent to a lab for analysis. You may feel mild cramping during the procedure, but it is generally safe and monitored closely.
Is the L/S Ratio Test Painful?
You may experience some discomfort or mild cramping during the procedure, but most women describe it as similar to a mild menstrual cramp. You will likely be able to return home the same day, but you may need to rest for a short time afterward.
How Often Is This Test Done?
This test is typically performed once during pregnancy, usually between weeks 34 and 37 of gestation, depending on your risk factors and overall pregnancy progress.
  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