Vasopressin not used as a uterine stimulant?

Hi all. In this pharmacology prac that we did, we tested responses of the rat uterus to oxytocin and related drugs. The response to vasopressin is essentially zero whereas the response to oxytocin is quite high. There is one question about why vasopressin is not used as a uterine stimulant, and we have no idea. Literature articles seem to say that vasopressin is more potent than oxytocin in the early stages of pregnancy, but less potent in late pregnancy. Is this related to the question? If so, in what way?