How can you tell if the baby is back to back?

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The head can bob in and out, although normally not that common in 1st. If the midwife can feel and give an idea to size they might be able to tell positon, but that doesn't mean baby won't move.
Mine was back to back.

if you lie on your back and have a feel of your tummy you will be able to feel limbs in the middle and the belly button will dip in. this is if your baby is bk to bk

I had lower back /hip pain / buttock ache for 2wks before my OP baby came out. Am watching for signs with this one (am also 34wks) - have had it a couple of times lately, I think this is when the baby has spun into that position but he seems to spin out again after a hot bath!