Question about obstetrics and obstetricians.?

I am doing an ignment and i would like to know if anyone has any info in the obstetric model of maternity care. Do pregnant women see the same doctor/obstetrician for every prenatal checkup like in the midwifery model or do they see whoever is on call when they go? If it is the same doctor for all the visits then does that same doctor deliver the bay or, again, is it whoever is on call? Can someone help me?