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Lecithin-Sphingomyelin Ratio (Amniotic Fluid)

Does this test have other names?

L/S ratio

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, chemicals made by the lungs that allow them to work properly. Without surfactants, the small air sacs in your lungs, called alveoli, would collapse, preventing oxygen from entering the bloodstream.

In the last three months of pregnancy, your fetus' lung surfactants can freely pass into the amniotic fluid. Once the surfactant becomes part of the amniotic fluid, your doctor 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 doctor 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 doctor doesn't know exactly how many weeks pregnant you are. Your doctor probably won't order this test if your baby may be born at less than 32 weeks, because 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 doctors 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:

  • Gestational diabetes

  • High blood pressure in pregnancy

  • Your water breaks early, a condition called premature rupture of amniotic membranes

  • Placental insufficiency, meaning that the placenta can't fully support the developing fetus 

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 doctor 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 health care provider.

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 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 doctor may want to see a different result. If you have diabetes or kidney disease, for example, your doctor 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 two ways:

  • Amniocentesis. Your doctor 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 doctor 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 doctor about the amount and color of the fluid – whether it's clear, cloudy, or tinged brown or pink/red.