Yes, there are also special baby carriers for premature babies that transport the child in a horizontal position. In an upright sitting position, the upper body can collapse, which restricts breathing.
The Römer Babysafe carrycot can be recommended here, as it scored a good mark in the ADAC test. This carrycot is strapped to the middle of the rear seat. However, the child can only lie in the seat for a few months.