Why don't babies choke when they are on their back?

Babies are actually safer on their backs. When a baby is on his back, the air tube (trachea) is on top of the esophagus (the tube that carries food).  If a baby spits up while on his back, the food and fluid run back into the stomach and not to the lungs. When a baby is on his stomach, the esophagus (or food tube) is on top of the trachea and any food or fluid that is regurgitated or refluxed can more easily pool at the opening of the trachea, making it possible for the baby to aspirate or choke. The following images and video help illustrate this.

Image from National Institute of Child Health and Human Development Safe to Sleep® Campaign, available at: www.nichd.nih.gov/sts/news/downloadable/Pages/default.aspx