did u try limits on your pub med search b/c I found articles when I did it:
Here is one citation (can't give web sites because it is thru my school):
An open-label, multicentre trial to evaluate the vaginal bleeding pattern of the combined contraceptive vaginal ring NuvaRing.Obstetrics and Gynaecology Clinic, Careggi Hospital, University of Florence, Florence, Italy.Eur J Obstet Gynecol Reprod Biol. 2008 Jul;139(1):65-71. Epub 2008 Mar 21.
I limited it to clinical trials 1,2,3