Lecithin-Sphingomyelin Ratio (Amniotic Fluid)
Does this test have other names?
What is this test?
This test measures the amount of two substances, lecithin and sphingomyelin, that
are found in the amniotic fluid during pregnancy. The two substances are surfactants.
These are chemicals made by the lungs that let them to work properly. Without surfactants,
the small air sacs in your lungs (alveoli) would collapse, preventing oxygen from
entering the bloodstream.
In the last 3 months of pregnancy, your baby's lung surfactants can freely pass into
the amniotic fluid. Once the surfactant becomes part of the amniotic fluid, your healthcare
provider can measure it.
In a normal pregnancy, the concentration of lecithin in the amniotic fluid continues
to rise. But the concentration of sphingomyelin remains about the same throughout
the pregnancy. Your provider will compare the concentration of each surfactant to
find out how mature your baby's lungs are. The higher the lecithin concentration compared
with the sphingomyelin concentration, the more likely the lungs are mature.
Why do I need this test?
You might have this test if you're pregnant and expected to deliver before 39 weeks
or your healthcare provider doesn't know exactly how many weeks pregnant you are.
Your provider probably won't order this test if your baby may be born at less than
32 week. At that point, his or her lungs will be immature regardless of test results.
In a developing fetus, the lungs are the critical factor in finding out whether a
baby is ready for life outside the womb. Many healthcare providers use lab tests to
predict how mature the baby's lungs are before delivery.
You may be at risk for early delivery if you have any of these conditions:
High blood pressure in pregnancy
Your water breaks early (premature rupture of amniotic membranes)
The placenta can't fully support the developing fetus (placental insufficiency)
Rh disease (erythroblastosis)
What other tests might I have along with this test?
You may have this test as part of a procedure called amniocentesis. In amniocentesis,
your healthcare provider collects amniotic fluid to measure several other lab values.
What do my test results mean?
Many things may affect your lab test results. These include the method each lab uses
to do the test. Even if your test results are different from the normal value, you
may not have a problem. To learn what the results mean for you, talk with your healthcare
The results of this test are given as a ratio of lecithin to sphingomyelin:
A value of less than 1.5:1 means that your baby's lungs are immature. If born now,
your baby may have breathing problems.
A value between 1.5:1 and 1.9:1 means that your baby may be at risk for immature lungs
and breathing problems.
A value of greater than 2:1 means that your baby has mature lungs and is ready for
life outside the womb.
In some cases, your healthcare provider may want to see a different result. If you
have diabetes or kidney disease, for example, your provider may want the value to
be greater than 3.5:1 to make sure that your baby's lungs are mature.
How is this test done?
This test requires a sample of amniotic fluid. Amniotic fluid can be collected in
Amniocentesis. Your healthcare provider will insert a long needle through the abdomen and into the
uterus to collect the sample.
Direct collection from vaginal fluid. If your water breaks, your healthcare provider can collect amniotic fluid that pools
in your vagina.
Does this test pose any risks?
If amniocentesis is performed, the risks of complications are rare but can include:
Leaking of amniotic fluid. Sometimes this can lead to infection, which can lead to
miscarriage early in pregnancy, or preterm labor and preterm birth later in pregnancy.
Injury to the baby if the needle touches the fetus.
Leaking of blood from the placenta into your own bloodstream. This can cause problems
for later pregnancies.
What might affect my test results?
Numerous factors can affect your test results. These include:
Presence of fetal or maternal blood in the amniotic fluid
Abnormally high volumes of amniotic fluid, which may give false-low test values
Presence of fetal meconium, which is fetal stool. This happens if the fetus has a
bowel movement in the womb.
How do I get ready for this test?
You don't need to prepare for this test. If your water breaks early, it's important
to tell your healthcare provider about the amount and color of the fluid—whether it's
clear, cloudy, or tinged brown or pink/red.