subchorionic cyst is a placental cyst from the fetal side,as you know the placenta has 2 sides maternal and fetal side which is facing the fetus,any way its of no significance as i ve been told by my doctor and thats what i read in many sites also,but they added that if it attained large size or sited near umbilical cord insertion they may cause intrauterine growth restiction but this is not comman,any way i prefer you consult your obstetrician for assurance.
wish u a healthy baby.
Chorionic is related to the placenta, I believe. So it might be a placental cyst. I don't think I've ever heard of that either! I'm sorry I don't have any information.