Your wife sounds just like me!
Your wife needs to go to nursing school, obtain her RN, and while in school it would be nice if she had some OB clinical experience; post-partum or even prenatal class teaching experience (the school will determine what experiences she will have, but tell her to always look and ask hospitals or even a working RN if she would agree to train her during clinical time, etc and work this out with the school).
I was a new grad and hired directly into Labor and Delivery, and loved it. I was very determined to get hired and work there, so I did everything I could to ensure that my chances were the best they could be.
I have worked other areas but I always go back into that OB. It's where my heart is.
Once your wife IS a Labor and Delivery nurse, she can then become certified in fetal monitoring, and other areas - but since OB can be very overwhelming, I would advise her to wait and just learn the basics.
Best of luck to her and you - and keep us updated!