It could work in your favor to work in a hospital first, because you could then transfer from whatever unit you start on to OB. That said, it might take more time for you to get hired in OB, firstly because if you got hired on say, cardiac or ortho or something it would be expected, or at least it would be classy to stick around for a year or so before zipping off to another unit or another hospital. In fact, a lot of managers would probably frown on someone who worked on their respected unit for 6 months and then split. Secondly, you'd have to wait for an opening at the right time.
What I would suggest, whatever route you go is to think about why you want to be an L&D nurse. I'm not saying it as a challenge or anything, I'm just saying that recalling my interview for postpartum, the manager didn't want to hear "fluff" like "oh I just love babies!" (Well DUH!) Or "I want to be able to hold a mother's hand through labor..." yes of COURSE! But pay more attention to the hospital itself and the clientele it serves- an example: my hospital is inner-city. 85% or more of our patients are living in poverty, many are from broken homes, many are teens, and a large population are either Hispanic, Hmong or Somali (bi-lingual skills are naturally a huge plus). I find people fascinating, and I find the diversity of child birthing practices really interesting. I have a soft spot for teen-mothers, I feel like if I as a nurse can make a mom feel safe in the hospital and reassure her that her "babydaddy" isn't going to get past the front doors, then good. I like to feel useful to someone.
That wasn't meant to sound like a "yay for me" sermon, but you get the idea. My manager was real upfront that people think they want to go into L&D because it's going to be just like "A Baby Story" or some other TLC show. So I'd say if you have a specific hospital in mind, research it. Know why you want to work in L&D (also be willing to start in Postpartum or Special Care Nursery). I agree with joining AWHONN. It looks good on a resume. It shows that you are willing to learn about this specialty on your own. Becoming an IBCLC is a pricey investment but if you have the resources then I'd suggest doing it. Don't bother with getting certifications- they'll train you and odds are you won't pay a dime whereas on your own you might pay...several dimes... plus, some certifications require that you've already worked in the field for X number of hours.
I got my foot in the door where I'm at because I had both my kids at this hospital- the nurse manager was working the floor one day and was wheeling me down to the special care nursery to see my baby. I remembered her name. I came back a few days after discharge with a resume (and another thank you ). They weren't hiring at that time, so in the meantime I got a job on the cardiac floor, floated all over the place for a year and then applied to a special care nursery job, the recruiter called me and said that I wouldn't be considered for that because I had no experience, HOWEVER- there's a postpartum job that's not posted yet that I fit many of the qualifications for and my cover letter sounded like I'd do well there. So...I applied, interviewed and got hired. Right place, right time...