We have been advised not to use one. DD has hip dysplasia, and the harness seat that the baby sits in is akin to the Baby Bjorn carriers. There's a diagram I've seen, it puts too much pressure on the hip and can cause it to either dislocate (partially or fully) or set in the wrong position. The legs need to be far more spread than the jolly jumper, BB etc seats allow. Our doctor did say that lots of standing and pressure is good, just not in a jolly jumper. Seeing as our next babies will be at risk of DDH, we won't use them.