Amniotic Lecithin-Sphingomyelin Ratio

Your caregiver has ordered an amniotic L/S ratio. This is a test done on amniotic fluid (the fluid the baby floats in within the uterus). This fluid is removed from your uterus with a needle. This test is done to test the maturity of the baby's lungs.

This test helps to determine if the baby will have breathing problems from under-developed lungs (hyaline membrane disease, respiratory distress syndrome), if delivery occurs near the time this test is done. It helps to prepare for lung problems in the baby and helps your caregiver know what steps may be needed at the time of delivery, and if it is safe to deliver the baby at this stage in the pregnancy.

If you are Rh negative you may need to have additional testing following the amniocentesis (the drawing of the amniotic fluid) to make sure you have not been sensitized to your baby's blood. Rhogam may be given following an amniocentesis to make sure sensitization does not occur. Occasionally testing is done prior to giving Rhogam to determine if it is needed.


Your caregiver will give you instructions prior to this test. This test can usually be done any time of day on any diet without fasting. Follow instructions of your caregiver and keep appointments as recommended.

  • Tell the person doing the test if you have been told of any pregnancy difficulties, such as early labor, a weak cervix, a placenta that is in an unusual position, a placenta that is separating from the wall of the uterus too soon and if you are Rh Negative

  • If ultrasound is used, you may need to drink extra fluids so you have a full bladder for the test


  • Weeks' Gestation: 15

  • Amniotic Fluid volume: 450

  • Weeks' Gestation: 25

  • Amniotic Fluid volume: 750

  • Weeks' Gestation: 30-35

  • Amniotic Fluid volume: 1500

  • Weeks' Gestation: Full term

  • Amniotic Fluid volume: Less than 1500

  • Amniotic fluid apperance: clear; pale to straw yellow.

  • Lecithin/sphingomyelin (L/S) ratio greater than 2:1.

  • Bilirubin: less than 0.2 mg/dL.

  • No chromosomal or genetic abnormalities.

  • Phosphatidycerol (PG): positive for PG.

  • Lamellar body count greater than 30,000.

  • Alpha -fetoprotine: dependent on gestational age and lab technique.

  • Fetal lung maturity (FLM).

  • Mature: greater than 260 mPOL.

  • Transitional: 260-290 mPOL.

  • Immature: greater than 290 mPPOL.

Ranges for normal findings may vary among different laboratories and hospitals. You should always check with your doctor after having lab work or other tests done to discuss the meaning of your test results and whether your values are considered within normal limits.


Your caregiver will go over the test results with you and discuss the importance and meaning of your results, as well as treatment options and the need for additional tests if necessary.


It is your responsibility to obtain your test results. Ask the lab or department performing the test when and how you will get your results.