Think about the flexion forces placed on the back during pregnancy and carrying a baby in front rather than on her back. Loading on her back is least likely to cause pain in the spine out of the possibilities (carrying baby on hip or in front) as it is a weight carried closest to the axis of the spine.

Have a look at some back care when lifting sites, they should give you some ideas of the forces of carrying loads in different body positions. Ask her WHY she thinks carrying the baby on her back will cause pain. Education can provide a lot of reassurance.