I really feel you on this, being a first time mum and hearing that news from your gynae must have been quite worrying. My girl also had an anterior placenta and my gynae told me the same thing, but we managed to get some decent 3D scan images at around 28 weeks because the baby had shifted to a better position by then. It really does depend on how baby is lying on the day, so I would say just go ahead and try the scan closer to 28 weeks and keep your fingers crossed that baby cooperates lah.