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Lecithin/sphingomyelin (l/s) ratio & phosphatidyl glycerol (pg) tests

The lecithin/sphingomyelin (l/s) ratio and phosphatidyl glycerol (pg) tests are two ways of measuring the maturity of your baby's lungs. Used in combination, they give the doctor a unique ability to determine whether a baby will be able to breathe after delivery, or whether it will need the aid of a respirator.

Respiratory distress syndrome, also called hyaline membrane disease, is a common problem that affects premature babies. A chemical called surfactant is necessary to keep the membranes of the lungs from sticking together. Its purpose is similar to flour on a countertop when rolling out pastry, and enables each breath to expand the lungs and fill with air. If a baby is premature, there may not be enough surfactantA substance present in the lungs to help keep them inflated. Surfactant consists of certin lipoproteins that reduce the surface tension of the fluid in the lungs permitting gas exchange in the aveoli and contributing to the elasticity of the lung tissue. Premature babies often lack sufficient development of surfactant at birth to breathe without assistance.
Visit our comprehensive glossary for more pregnancy terms and definitions.
, and the baby will need to be placed on a respirator.

In the early 1970s, scientists developed the L/S ratio and PG tests to help doctors measure the maturity of the baby's lungs before it is born. The results of the test can help them to decide whether to proceed with the delivery of the baby or consider another course of action.

How l/s ratio & pg tests work

In order to measure the levels of lecithin, sphingomyelin and phosphatidyl glycerol, an amniocentesis will be required. As the lungs mature, the levels of lecithin rise and those of sphingomyelin decrease. Both these blood factors are found in the amniotic fluidThe liquid, which is produced by both the fetal membranes and the fetus that surrounds the baby during pregnancy. The liter of fluid at term serves to protect the fetus during pregnancy and also provide active chemical exchange.
The amniotic fluid consists of maternal and fetal plasma in varying concentrations. The pH of the fluid is almost neutral and clear, although lipids and desquamated fetal cells can make it cloudy.
Visit our comprehensive glossary for more pregnancy terms and definitions.
, and the ratio between them will indicate the maturity of the lungs.

The amniotic fluidThe liquid, which is produced by both the fetal membranes and the fetus that surrounds the baby during pregnancy. The liter of fluid at term serves to protect the fetus during pregnancy and also provide active chemical exchange.
The amniotic fluid consists of maternal and fetal plasma in varying concentrations. The pH of the fluid is almost neutral and clear, although lipids and desquamated fetal cells can make it cloudy.
Visit our comprehensive glossary for more pregnancy terms and definitions.
will also reveal the presence of phosphatidyl glycerol. Your doctor will look for a positive result to confirm that the baby will not suffer from respiratory distress syndrome on delivery.

Who l/s ratio & pg tests are designed for

The test is administered if there is a risk the baby will be born before the lungs have had a chance to mature. If the lungs are sufficiently mature, a premature baby can be born with little risk of developing respiratory distress syndrome.

Risks associated with l/s ratio & pg tests

The L/S ratio and PG tests analyze the amniotic fluidThe liquid, which is produced by both the fetal membranes and the fetus that surrounds the baby during pregnancy. The liter of fluid at term serves to protect the fetus during pregnancy and also provide active chemical exchange.
The amniotic fluid consists of maternal and fetal plasma in varying concentrations. The pH of the fluid is almost neutral and clear, although lipids and desquamated fetal cells can make it cloudy.
Visit our comprehensive glossary for more pregnancy terms and definitions.
and are in themselves not risky. However, an
amniocentesis is required with the attendant risks that it entails.

When l/s ratio & pg tests are performed

The test is performed between week thirty four and week thirty eight. The L/S ratio becomes effective after week thirty four. A baby born earlier than this will need the assistance of a respirator to breathe. After week thirty eight the baby is no longer premature and should have developed sufficiently to breathe on its own.

If the test results are positive

Your doctor will decide how to proceed depending on the results of the test. Ideally the delivery of the baby will be delayed so that the lungs can mature in utero. If this is not possible your doctor will measure the levels of surfactantA substance present in the lungs to help keep them inflated. Surfactant consists of certin lipoproteins that reduce the surface tension of the fluid in the lungs permitting gas exchange in the aveoli and contributing to the elasticity of the lung tissue. Premature babies often lack sufficient development of surfactant at birth to breathe without assistance.
Visit our comprehensive glossary for more pregnancy terms and definitions.
in your baby's lungs immediately after delivery. It is now possible to introduce surfactant directly into the lungs of a newborn, allowing it to breathe on its own without the assistance of a respirator.