What structural factor causes reflux in infants before maturation?

Lower esophageal sphincter (LES)

In infants experiencing reflux, the underlying physiological reason is often the immaturity of the lower esophageal sphincter (LES). The LES is the critical muscular ring located between the esophagus and the stomach; its function is to open to allow food passage and then close tightly to prevent stomach contents from flowing backward. In many babies, this sphincter has not yet fully matured or developed the necessary strength and coordination. When this muscle remains too relaxed or opens inappropriately, it allows stomach contents, including acid, to flow back up into the esophagus, resulting in reflux or spitting up.