For the first time you must stay very aware of her condition, don't expect her to remember a safe word if she is too far along, or paniced. Ask her how she is doing often. Keep it low key and tell her that her safety and well being are your greatest concern.

I don't believe in letting a slave top from the bottom but with a first timer you have to allow that. She will understand if you tell her this in advance. You will still have the pleasure of taking her where she has never been before.

Add to each session and take away more of her control each time as you learn her physical signals. PLEASE REMEMBER she will forget to use her safeword and that is almost a guarantee, take no for no at first, ask often and enjoy.