Rear facing infant carriers causes oxygen desaturation in new borns. Oxygen desaturation is when the blood oxygen levels fall possibly to the extent that not enough oxygen gets to the vital organs and in particular to the brain. In some cases this may cause Apnea (a breathing/sleeping disorder) or Bradycardia (the slowing of the heart). Some experts believe that both Apnea and Bradycardia have links to cot death and one medical research report has now made this link. Others believe that the lowering oxygen levels can cause problems in babies development and /or learning difficulties in later life and again there are several medical research reports which makes this link. Whilst there is certainly some debate on the effects of oxygen desaturation numerous medical research reports have concluded that many newborns (and in particular low birth weight and pre-term babies) DO often suffer lowering blood oxygen levels when restrained in an infant carrier in the early weeks of life. The reasons for this may be varied but often include the facts that a new born can not hold their head (when the head "flops" the airway narrows) and that the lungs are less efficient if the chest diaphragm is restrained by the restraint straps. Additionally, when a new born baby is placed in a wedge shape the babys internal organs become squashed pushing into the lower area of the lung, the area in which a new born baby tends to breathe from. In the USA doctors recommend that babies should be observed in infant carriers before leaving hospital so to ascertain if they can travel in an infant carrier healthily. Sadly, however not all babies at risk can be identified and in the U.K. this procedure is not followed. At the bottom of this page there are details of some of the medical research reports. You can of course do your own research on the net try several search engines using a combination of the words oxygen desaturation, low blood oxygen levels, car seats.