Don't sell yourself short. Your Med/Surg experience is valuable in pursuing a L&D job. You have strong skills and a knowledge base which will enable you to care for pregnant patients with a wide range of medical conditions. You are skilled in the care of post-op patients, which, of course, includes C-section moms. You have time management and prioritization skills which are extremely important in L&D.
If your hospital won't allow you to transfer directly into L&D then you might want to consider High Risk Antepartum, Mother-Baby, Post-Partum, Nursery or NICU, any of which would be a good stepping-stone into L&D. You say that you are in a major hospital, which is good, because bigger hospitals can typically offer a more thorough orientation. Make some friends on the L&D unit so you will have an "in" when a position opens up.
Good luck and be persistent! I have never wanted to do anything other than Maternal-Child Health, and I thank my lucky stars that I've never had to!