A polymorph Form III of 4-(2-(4,4-dimethyl-2-oxooxazolidin-3-yl)thiazol-4-yl)benzonitrile and methods of preparing Form III are described. Also provided are methods of contraception, treating or preventing fibroids, uterine leiomyomata, endometriosis, dysfunctional bleeding, polycystic ovary syndrome, and hormone-dependent carcinomas, providing hormone replacement therapy, stimulating food intake, and synchronizing estrus including using polymorph Form III of 4-(2-(4,4-dimethyl-2-oxooxazolidin-3-yl)thiazol-4-yl)benzonitrile. Also provided are methods for preparing polymorph Form I of 4-(2-(4,4-dimethyl-2-oxooxazolidin-3-yl)thiazol-4-yl)benzonitrile from polymorph Form III of 4-(2-(4,4-dimethyl-2-oxooxazolidin-3-yl)thiazol-4-yl)benzonitrile.