Meconium is the dark, tarry contents of the fetal gastrointestinal tract. Meconium passage at labor and delivery is common and not a sign of fetal distress unless it is accompanied by other ominous signs.
Meconium release into the amniotic fluid is however considered a sign of intrauterine fetal distress. This can be caused by placental insufficiency, maternal hypertension, and preeclampsia, among other things.
When meconium is freshly released, it is clumpy and oats in the amniotic fluid. Within hours, it distributes particulate matter throughout the sac. Macrophages in the fetal skin, mucus membranes, and the amniotic sac phagocytize the particles, giving a green cast to the tissues.
After several days, the fluid is greenish-brown but clear, and the fetus and membranes are stained. After several weeks, the fetal skin and fluid will clear, but the amniotic membranes, fetal mucus membranes, and nail beds remain stained.