Green-stained amniotic fluid, often referred to as meconium-stained amniotic fluid, is present in 5% to 20% of patients in labor and has been traditionally considered an obstetrical hazard. Discolored amniotic fluid has been attributed to the presence of heme catabolic products from the passage of fetal colonic content (meconium), intraamniotic bleeding, or both. The frequency of green-stained amniotic fluid increases as a function of gestational age, reflecting maturation of the gastrointestinal system, and reaches approximately 27% in postterm gestation.

Before the introduction of routine continuous fetal heart rate monitoring, green-stained amniotic fluid during labor was associated with fetal acidemia (umbilical artery pH <7.00), neonatal respiratory distress, and seizures, and was considered a risk factor for cerebral palsy. Hypoxia has been considered the main mechanism responsible for fetal defecation and meconium-stained amniotic fluid; however, most fetuses with meconium-stained amniotic fluid do not have fetal acidemia. Nonetheless, in the absence of fetal heart rate abnormalities, meconium-stained amniotic fluid is not associated with fetal acidemia. Intraamniotic infection/inflammation has emerged as an important factor in meconium-stained amniotic fluid in term and preterm gestations, and green-stained amniotic fluid is a risk factor for maternal and neonatal infections. Whether intraamniotic infection/inflammation results in discoloration of amniotic fluid via oxidative stress or the passage of meconium has not been determined. Two randomized clinical trials suggest that, in patients with meconium-stained amniotic fluid, intrapartum administration of antibiotics decreases the rate of clinical chorioamnionitis. Meconium aspiration syndrome is a severe complication typical of term newborns, which develops in 5% of cases presenting with meconium-stained amniotic fluid. Meconium aspiration syndrome is attributed to the mechanical and chemical effects of aspirated meconium coupled with local and systemic fetal inflammation. A systematic review of randomized controlled trials suggested that amnioinfusion may decrease the rate of meconium aspiration syndrome. Routine naso/oropharyngeal suctioning and tracheal intubation in cases of meconium-stained amniotic fluid have not been shown to be beneficial and are no longer recommended in obstetrical practice. Histologic staining of the membranes with meconium has been used in the context of medical legal litigation to attempt to time the occurrence of fetal injury. This has been largely based on the results of in vitro experiments. However, extrapolation of these findings to the clinical setting is unwarranted. Experimental studies in animals and observational studies in human fetuses suggest that fetal defecation is a physiological phenomenon throughout pregnancy.