Hello, sistermike,
RNC = Registered Nurse Certified
Many on here are certified in Labor and Delivery, In Patient OB, EFM (electro-fetal monitoring), etc.
It is a way of showing added excellence in a specialty area. Has no bearing on abilities or difference in skills. It is an honor to pass these national certification exams. Some hospitals will pay for this added certification. Also, the individual will be required to obtain continuing education hours to keep the certification.
Now, there are so many certifications not only for RN, but, LPN, too. They all carry their own initials of designation.