It is highly recommended that you have a detailed discussion of the risks benefits with your doctor, since your detailed medical / Obstetric history and clinical findings are important. Medically, a low lying placenta is referred as a total Placenta Previa. The key concern regarding this particular circumstance is that some of the placenta is over an opening and not directly and intimately attached to the uterine wall. During flying there is a pressure decrease in the plane at normal altitude and this may act as suction on this small part of the placenta and can potentially induce bleeding. While this is not a frequent occurrence early in the pregnancy, it is still an unnecessary additional risk and a factor in the decision making. Furthermore, this type of placenta may occasionally bleed spontaneously. As the pregnancy progresses, many such placenta previas tend to self resolve during the placental growth and in that case your doctor may change the recommendations as the pregnancy continues.