I had an anterior placenta with DD, and felt movements from around 19w onwards, DH didn't feel them to much later. I also worried a couple of times in the 3rd trimester, because hadn't felt DD for a few hours, but each time, when the hooked me up to check at hosp, she was fine and kicking - the monitor picked it up, but I couln't feel it, so the mw reckons she was probably kicking into my placenta.

This time around my placenta is posterior and I felt movements much ealier, DH also felt wiggle at around 19wks which shocked us both!

DD was also posterior, and I'd heard the same as NikW, in that some think because they prefer to face the placenta, it might cause them to be posterior, but there are also other things that cause OP's so its nothnig to worry about