welcome to HealthcareMagic
During 7th week the placenta on anterior side is called low lying placenta and in most cases the placenta migrates to the upper side of uterus
by 38 weeks of pregnancy
.But if it remains on lower side covering the canal by 38 weeks this condition is called placenta previa
.In such cases there can be few problems at the time of delivery.Right now you should avoid sex and see the position of placenta in next scan and if it has shifted to upper side by that time you can have protected sex at that time till 7 months of pregnancy.