The same thing applies to labor. Your midwife will make herself available at whatever point in labor you feel you would benefit from her support. A doctor will monitor the progress of your labor and will check in and out with you as labor progresses, but typically won't be spending a lot of time with you until the moments before the birth. This is not to say that one is necessarily better than the other. You have to decide which approach to birth is most comfortable to you, and which caregiver is going to meet your medical needs.
Some midwives are nurses who have done some additional midwifery training.
Others have been trained by other midwives, and the legality of midwifery in various states varies. Most midwives, other than midwives affiliated with particular hospitals, will attend home births. In other words, if you're a midwife on staff at a large hospital, you will probably do all your practicing within the hospital. The American College of Obstetricians and Gynecologists does not routinely recommend prenatal supplements to all pregnant women. What they do recommend, however, is that all women of childbearing age consume at least 0.4 mg of folic acid.