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The lecithin-to-sphingomyelin ratio (L/S ratio) is one of several methods used to assess fetal lung maturity (FLM). First introduced in the 1970s, this biochemical test requires amniotic fluid collection via amniocentesis to evaluate the risk of respiratory distress syndrome (RDS) in neonates. Thin-layer chromatography (TLC) is then used to measure lecithin relative to sphingomyelin.[1] Historically, clinicians have relied on this test to guide the timing of delivery before 39 weeks of gestation in an effort to reduce the risk of RDS. In recent years, the use of this test has declined due to guidelines and recommendations from major medical societies.[2] Fetal lung development is a continuous process in which pulmonary maturation progresses with increasing gestational age. Lung development occurs in 5 sequential stages: embryonic (3-7 weeks), pseudoglandular (5-17 weeks), canalicular (16-26 weeks), saccular (26-36 weeks), and alveolar (32 weeks through childhood).[3] The most significant maturation occurs during the alveolar stage when type II pneumocytes develop. These cells produce surfactant, which is essential for reducing alveolar surface tension and maintaining lung function.[4] Surfactant prevents alveolar collapse during expiration. This mixture consists of phospholipids, proteins, and lipids. Of particular importance is the phospholipid composition. Phosphatidylcholine, also known as lecithin, is one of the phospholipids present in mature surfactants. This substance is stored and secreted by organelles called "lamellar bodies."[5] Lamellar bodies appear after 22 to 24 weeks of gestation. Before the 28th week, the fetal lung primarily synthesizes sphingomyelin, a nonpulmonary lipid. At approximately 32 weeks, lamellar bodies increase, leading to greater surfactant production in the fetal lungs and amniotic fluid. At this stage of lung maturity, lecithin and sphingomyelin are present in relatively equal concentrations.[6] Mature surfactant is produced by 35 weeks of gestation, marked by a sharp increase in lecithin concentration in the fetal lungs and amniotic fluid. An L/S ratio of 2:1 or greater indicates FLM. Fetuses delivered before this gestational age face a higher risk of neonatal RDS.[7]

The birth of a neonate before 39 weeks presents substantial respiratory risks due to incomplete lung development. Major complications include RDS resulting from surfactant deficiency, bronchopulmonary dysplasia (BPD) contributing to chronic lung disease, and pulmonary hypertension impairing oxygen exchange. These neonates frequently require respiratory support, including oxygen therapy, mechanical ventilation, and surfactant replacement, to enhance outcomes.[24]