Because premies have weaker muscle control than newborns, sometimes when they are sitting up their chin drops too far into their chest and they cut off their own airway, and they don't have the muscle strength to lift their chin back up.  Most hospitals do a car seat study for babies born more than 4 weeks early to monitor their ability to maintain head control when in a sitting position.  The test simply entails putting the baby on a monitor and then place them in their carseat for 60-90 minutes and monitor their heart rate and breathing.  If they do not pass this test, rolled up baby blankets can be used to stablize their head, and if that doesn't work, some babies will have a car bed ordered for them so they can be in the laying position for all car rides.