The Fetal Lung Maturity Test (FLMT) is a specialized diagnostic procedure performed during the later stages of pregnancy. Its purpose is to evaluate the functional readiness of a fetus’s lungs prior to a potential early delivery. This assessment helps medical teams determine if the baby’s pulmonary system is developed enough to support breathing outside the uterus. The goal of this testing is to optimize the timing of delivery, balancing maternal risks against the baby’s readiness for life outside the womb.
Why Lung Maturity is Critical
The transition from fetal life to air breathing requires a fully functional pulmonary system. Lung maturity depends on the presence of pulmonary surfactant, a complex mixture of lipids and proteins produced by specialized lung cells. Surfactant’s main function is to lower the surface tension within the tiny air sacs, known as alveoli, preventing them from collapsing completely when the infant exhales.
Surfactant production begins around 20 to 26 weeks of gestation, but the necessary concentration develops later in the third trimester. Without adequate amounts, the lung tissue becomes stiff and difficult to expand, leading to Respiratory Distress Syndrome (RDS). RDS is a major contributor to mortality and illness in premature newborns.
Infants with RDS exhibit symptoms like rapid, shallow breathing, grunting, and chest wall retraction immediately after birth. The lack of proper gas exchange causes oxygen deprivation and carbon dioxide buildup. Assessing lung maturity is a predictive measure used to mitigate the risk of this serious breathing difficulty if an early delivery becomes necessary.
When the Test is Performed
The determination of fetal lung maturity is reserved for situations where delivery is considered before 39 weeks of gestation. The test is only performed when the results will directly influence the clinical management plan, such as deciding whether to proceed with delivery or delay it. Current guidelines have reduced the overall frequency of this test, as non-medically indicated deliveries before 39 weeks are discouraged due to associated neonatal complications.
This evaluation is necessary when a medical condition requires a preterm or early-term birth. Conditions include severe preeclampsia, uncontrolled maternal diabetes, or placental issues like placenta previa resulting in bleeding. Fetal problems, such as suspected intrauterine growth restriction (IUGR) or certain fetal distress issues, can also trigger the need for testing when delivery is planned soon.
The test may also be considered when a patient presents with threatened preterm labor or premature rupture of membranes, and the exact gestational age is uncertain. A confirmed mature result allows the medical team to proceed with necessary intervention without the added risk of major respiratory complications. If lung immaturity is confirmed, the team gains time to administer treatment to the mother to help the fetus.
How the Sample is Obtained and Analyzed
The sample required for fetal lung maturity testing is typically amniotic fluid, which contains phospholipids shed from the fetal lungs. The standard collection method is amniocentesis, usually performed in the third trimester. During the procedure, the physician uses ultrasound imaging to guide a thin needle through the maternal abdomen and uterine wall into the amniotic sac to withdraw the fluid.
The process carries a small risk of complications, including temporary cramping, fluid leakage, or, rarely, infection or preterm labor. If membranes have already ruptured, a sample may sometimes be collected non-invasively from the vaginal pool. However, amniocentesis provides the cleanest sample, as contamination from blood or meconium can interfere with laboratory test accuracy.
Once collected, the fluid is subjected to laboratory analyses to quantify surfactant components. The historical method is the Lecithin-Sphingomyelin (L/S) ratio, which measures the relative amounts of two phospholipids. Lecithin is the primary active ingredient in surfactant; its concentration rises significantly as the lungs mature, while sphingomyelin levels remain constant, forming the basis of the ratio calculation.
Newer, faster methods include the Lamellar Body Count (LBC) and the detection of Phosphatidylglycerol (PG). Lamellar bodies are storage organelles for surfactant within the lung cells, and their concentration in the amniotic fluid is measured by automated cell counters. PG is another phospholipid that appears several weeks after the lecithin surge, and its presence suggests advanced pulmonary development.
Understanding the Maturity Scores
The interpretation of fetal lung maturity tests provides the clinical team with an assessment of the baby’s respiratory risk. For the L/S ratio, a result of 2.0 or higher is considered mature in a non-diabetic pregnancy, indicating a low likelihood of developing Respiratory Distress Syndrome. An L/S ratio below 1.5 suggests a high probability of lung immaturity, while intermediate values may prompt further evaluation.
The presence of Phosphatidylglycerol (PG) in the amniotic fluid is an indicator of maturity, as this substance enhances the stability of the surfactant film on the alveoli. The Lamellar Body Count uses a numerical cutoff; counts above 50,000 per microliter suggest lung maturity, offering a rapid and objective result. Low counts, such as below 15,000 per microliter, point toward pulmonary immaturity and a high risk of RDS.
A mature result suggests that if delivery proceeds, the risk of developing RDS is minimized, allowing the medical team to focus on the reasons necessitating the early birth. This positive result provides reassurance but does not guarantee the maturity of all other fetal organ systems. The test is a predictive tool, not a guarantee of a complication-free outcome.
Conversely, an immature result triggers a management plan designed to accelerate lung development before birth. This usually involves administering antenatal corticosteroids, such as betamethasone, to the mother. Corticosteroids cross the placenta and stimulate the fetal lungs to increase the production and release of surfactant components.
The maximal effect of this steroid treatment is achieved between 24 hours and seven days after administration, which guides the timing of delivery. The test results guide the obstetrician’s decision to either postpone delivery, proceed with treatment, or move forward with the birth under close neonatal surveillance.