I would make an appointment with the manager of the Mother/baby unit and tell her your goal of working on this unit. Ask her what you could do to make this come true.
You could try by talking to the manager and see if they have extra hours or call hours in mother/baby where you could help out. Once you get your foot in the door, it is a lot easier. You would have experience in the department and have more value. That would make you eligible for extra training for the department. It is a lot of work but that's the fastest I know.