Final answer:
The obstetrician will inject oxytocin, also known as pitocin, to induce labor as it stimulates uterine contractions and facilitates childbirth.
Step-by-step explanation:
The hormone that the obstetrician will inject to artificially induce birth in Maria, who has been in labor for 12 hours, is oxytocin. Oxytocin is an endocrine hormone secreted by the posterior pituitary gland that functions during childbirth to stimulate uterine contractions. When labor needs to be induced, a pharmaceutical version of oxytocin, known as pitocin, is administered intravenously to enhance the contractions of the myometrium (the smooth muscle layer of the uterus) and promote the birth process.