I had bilateral cysts of 8cm in early pregnancy but in 7mnth sacn it has decreased to 3cm .what will happen ne

I was diagonised of central placenta previa in 6mnth and posterior previa in 8 mnth .what will happen to placenta in next 2 mnths .will a check in 36 mnth confirm normal or ceaserian delivery.Is ultra sound accurate in terms of detection of sound echoes.