The article doesn’t say why they develop breathing problems, only that they do (or rather that they are more likely to).

But having a baby staying in the car seat for hours on end is not the same as carrying a baby around in the car seat.

What’s the difference if someone is carrying the baby in the car seat for hours on end? As the article states: as well as being used for car transport, the seats are commonly used to carry around babies and to secure them in their pushchairs and even on [shopping carts].

Some of the new born babies developed breathing problems after an hour in the car seats and that some of the premature babies even stopped breathing for a short period.

Keep in mind that the caution is for babies three months and younger.

The study by doctors at the Children’s Hospital in St Paul and the University of Minnesota School of Medicine, also warned about not using other baby equipment which necessitated babies sitting upright during their first few months of life.

“In addition, the use of swings and other types of upright seating devices should be uniformly discouraged during the first months of life.”