Low AF volume has many causes, from low production to fluid loss due to rupture of the membranes. Since you are fine and did not report fluid loss from your genitalia we can dismiss that as a probable cause. Next we must consider low production due to kidney absence or malformations but that do not seem to be case either. Another cause is placental insuficiency but the baby is doing fine and growing as expected for a normal pregnancy. Being that said I would mention a seldom considered cause, that is, poor maternal hydration. Before considering that you ask for an appointment to perform Doppler studies of the placental-fetal circulatoon I would suggest you drink more water during the day, water, not juices and sodas, water. Within hours AF level could improve. Good Luck!