Out of the 4 of mine, my second was anterior at 38wks and was the only one that was anterior way before I went into labour (ROA at 38wks), #1 wasn't anterior at my last appointment, but was once labour started and my last two were born posterior.

Don't worry though if baby isn't fully OA (occiput Anterior - back to your belly) when labour starts. Some babies will get as far as LOA (Left occiput anterior - with back on left side) and will go the rest of the way when labour starts. And even then posterior babies can turn fully anterior during labour so it isn't that big a deal if they aren't at the start - it just means that you may have some 'back labour' until they do turn. And like the previous posters have said, check out www.spinningbabies.com for helpful tips on how to get baby to turn and to stay that way.