Caro, apparrently the place where your placenta attaches to your uterus is the place where the egg first lands in the uterus, so like Trish said, it can be different for each pg. And in answer to your question the scan after 30 weeks is to determine if the placenta has grown into my caesarean scar tissue.

I am told by my Doc birth is only made more difficult by an anterior placenta if it has embedded into the scar tissue. It can commonly cause bleeding in the third trimester if this happens which if it did would pretty mych rule out a VBAC. It also depends how deeply it has embedded into the scar, after labour, it may come away relatively easily, the problems mainly lie if it is embedded too deeply and has to be surgically removed which can cause huge amounts of blood loss and hysterectomy in worst case scenario.

I am not stressed at this stage, no point until after I have had scan, am a lot more relaxed this pg than last time, we live and learn I guess!