In my hospital, employees can only transfer to another unit if there is a job opening. You have to wait until the job is posted and then apply for it as an internal applicant. If the other unit is interested in hiring you, then they will contact you for an interview. If you are offered and accept the position, then you give notice to your manager, just like you would if you were leaving for another position.
But if you want to be a little proactive, here's some things you could try: 1. Do you know any nurses on L&D? You could ask if they are aware of any positions that will be opening up in the future. 2. Ask for a meeting with the nurse manager of L&D. Ask for her confidence, but express your interest in transferring to her unit in the near future. Ask her the best way to go about this (waiting for a position, submitting a resume, talking to HR, etc.) and if there is anything you can do in the meantime (NRP, fetal monitoring course, other education) that would make you a better applicant and help with your transition.