Re Checking further, I found that in madonna and child paintings, the baby is almost always cradled in the left arm. Could it be that the right is active because the left (where the heart lies) (1) is the vulnerable side, needing protection, and (2) comforts the babe with the rhythm of the heartbeat? How does that square with the experience of any lefties in our group who have raised babes?

Most women find that babies are quieter when cradled in the left arm, and yes the thought is it's easier for the infant to hear the mothers hear beat. i suspect lefties have a harder time of it, since there stronger hand is engaged hold the baby, and not free for other tasks.