A low lying placenta in early pregnancy is quite common. Those who have placenta near the cervical internal os, i.e. moth of the uterus, have a chance of bleeding from vagina. This can happen at sex, strenuous activity, esp lifting weighted objects, straining at stools, etc You need to be on guard and look for vaginal bleeding. If it occurs you should visit the gynecologist and get treated. AS the pregnancy advances the placenta generally moves up and at 9 months it may not lie near the internal os. But these things may not happen in some and these patients who have a low lying placenta after 20 weeks are termed Placenta previa and if it remains low tlll 9 months, it may cause some difficulty, like bleeding, esp before and after delivery.
YOu just need to observe if there is any bleeding or not. Avoid strenuous work, intercourse and vaginal drugs. In most of the cases as pregnancy advances the placenta moves upwards. That means it's quite unlikely that you will have low lying placenta in advanced pregnancy