That's about it. Being a registered nurse is necessary to carry the title of CNM but I believe CNMs can obtain their midwifery training in a variety of ways. Most common, probably, is to enter a midwifery program that is affiliated with a nursing program.
There is another way to become a midwife, though, besides through nursing or solely through apprenticeship, and that is to go to a school of midwifery (which also involves apprenticeship.) Midwives who take take the last two routes can also choose to be licensed or non-licensed.