Rowan, or Mountain Ash, with its red berries, is also called the Quicken Tree or Quickbeam. Quick is an old word for life, as in "The Quickand the Dead" . "Quickening" is the term used to describe the first feeling of movement of a baby in the womb , the first sign of life. So Rowan is the tree of new life.
The name Rowan is derived from the Norse word Runa which means "a charm". Every new life is precious and must be protected, so an amulet of a cross or a hoop of Rowan twigs tied with red thread - the colour of life - and hung on the cradle will protect a baby, and prevent it being stolen by the fierce folk (fairies)