Imam Jafar as-Sadiq [a.s.] has advised mothers that they should put their new born babies to sleep on their left side.
Unborn babies are not only used to hearing their mother's heartbeat, but their very existence depends upon them. Heartbeats mean to them a constant supply of food. Stoppage of heartbeats signals starvation and death. They depend so much upon the heartbeat that even after they are born, they become restless, if they do not hear it. A new-born knows its mother's heartbeat quite well and that is why it sleeps comfortably and peacefully, when it is on the left side of the mother and can hear the heartbeats clearly.
