Hmmmm, most do the opposite. I have several coworkers who went from OB to ER. I am currently a L&D nurse, but love ER as well. I actually finished posting on one of the new ER threads. I started out in the ER, but the ER I started in was a nightmare (low staff, unsafe pt loads, no preceptor, etc...) that I went back to L&D where I already had a niche (I was an OB scrub tech for years prior to nursing school
I absolutely love L&D, you probably will as well. ER RN's, in my opinion would make awesome OB nurses, due to the emergency situations that can arise on a regular basis. It is a bit different, we have pt with increased BP's but it is PIH, and is treated with different meds if the liver is involved, etc.
OB is a great place to be, it is just a bit different. The doc's are different, you will bang your head against the wall a bit more. But it really is worth it.