Babies should not be in carseats for prolonged periods of time unnessarily. I was advised by a peadiatrician that longer than 20mins should be avoided unless on a long journey in a car doing the primary job that they are designed for. Even then regular breaks of at least 10mins for every hour travelling should be taken. There is I believe medical data to back up the fact that some babies that have been sat in a car seat for as little as 30 minutes have less oxygen in their blood. all babies affected by reduced oxygen whether low birth rate or not. Whether this is a problem (short or long term) for all babies is unclear supporting info here with medical citations. Whether this is where the 2hr rule comes from or not...it certainly supports it (supports the 20min rule IMO)
Keeping a baby in a car seat unnessarily is a risk factor for SIDS. There chest and windpipe can become crushed by the weight of the babies own head and body. It doesn't just affect premature babies either....although some babies are at higher risk than others for unknown reasons.
I have seen my baby go blue in a car seat and I was petrified . It is a real risk. It is a life threatening risk.
Car seats are not intended to convert a forward facing pushchair to a rear facing one. Sleeping babies should be reomved from their seat when you get home.
WRT to RI's comments....
"Just playing devil's advocate - common sense could also tell you that they have just spent nine very happy months all scrunched up that this is their preferred state - as evidenced by the success of calming babies by tight swaddling and the comfort and security afforded even to adults by adopting the foetal position."
2 issues with this....the baby is swathed in fluid, which counters the effect of gravity. Secondly babies don't breathe in utetero so cot death is not a risk in the same way.
Its just that this two hour rule seemed completely arbitrary..... (see link and statements above)
And I realise you are focussing on Plagiocephaly, but it doesn't matter what the theories are behind the "2hour rule" or advise against doing it it whatever....plagiocephaly is hte least of the worries. It is entirely possible that the rule was "intrduced" for 1 reason...but is mis-communicated, and has evolved to apparently relate to the plagio...where it never was IYSWIM.
In summary, my thoughts are - a car seat is to be used to safely transport a baby in a car. A pushchair/pram is used to safely transport a baby on land. The 2 are not interchangeable to my mind. If I was the OP I would be looking at getting a pushchair/stroller/whatever that its suitable from birth.