It depends on how far you are in your planning. If it is something you have not put effort towards it is probably best to leave that off. If you have put effort, maybe 1- sentence saying what youve done in order to obtain your CNM.
If its a cover letter detailing which 2 of 3 units you can work.. L&D, Post-Partum & New Born Nursery then its ok. You want to keep it short and simple.. Do not add in things like "future plans" unless you are taking classes or in the process of making those plans happen.
The only thing I have done towards CNM school is applied at Frontier. Now I just have to wait and see what happens. I was torn about using it or not. I thought on one hand, it would show dedication. On the other, going to school for CNM may affect my work.
I have a question that is somewhat related to the OP's question. I am a nurse-midwifery student who is not working in L&D. I believe one of the hospitals I have applied to does not use CNMs. A question that often comes up in interviews is "What are your plans for the future/where do you see yourself in five years?" At a hospital that doesn't use CNMs, would it help or hurt to say that I hope to be working as a CNM?