obs are trained in surgery and are your best bet if you are facing complications.
certified nurse midwives are registered nurses who have received extensive training and pass a certification test.
certified midwives get the training and pass an exam, but have no other medical background.
I love my midwife and am all for natural birth (in fact, I'm trying it with my twins next month), but make sure you research a certified midwife as they aren't as tightly regulated usually
I'm using a midwife and she's great. What everyone said above is true. If you're not a high risk, midwife is perfectly fine. At the same time, if any complications occur, a midwife will refer you to an obgyn or take you to the hospital (if you have a home birth). Some midwives are nurses and some work in hospitals or have an office. Some do appt at your home.
I love midwives. They spend a lot more time with you for appointments and during labor. They are more hands on and they aren't big on interventions unless absolutely necessary.
Midwives vary greatly. Some work with Dr's and have hospital privileges and others don't and only do home births. Most Dr's are hesitant to take new patients past 20 weeks. I moved at 24 weeks and even with my full records was struggling to find a dr. The Dr that finally took me did so after I dropped names of two of her current patients that recommend her. Sometimes Dr's that specializes in high risk pregnancy are more willing to take on patients that are farther along. If you still can't find someone then call your insurance company, sometimes they can pull strings and get you a dr that would normally turn you away to take you.
An obgyn deals with the medical side of things like high risk or other complications during pregnancy... A midwife takes a more holistic approach to child birth and prenatal care. If you see a midwife and end up developing complications you will have to see an obgyn.