Amniotic band syndrome (ABS): a syndrome of birth defects that occurs when strands of the amniotic sac become loose and wrap around parts of the baby in the womb. The resulting compression from these amniotic bands leads to loss of blood flow to the affected areas and causes birth defects in the face, arms, legs, fingers, or toes. The arms and legs are the areas most commonly affected. Symptoms and signs of amniotic band syndrome vary widely depending upon the number and location of amniotic bands. The reason for the formation of amniotic bands is not well understood.