High adherence to intermittent and continuous use of a contraceptive vaginal ring among women in a randomized controlled trial in Kigali, Rwanda by Kestelyn, E, Van Nuil, JI, Umulisa, MM, Umutoni, G, Uwingabire, A, De Baetselier, I, Uwineza, M, Agaba, S, Crucitti, T, Delvaux, T, van de Wijgert, JHHM