As the mother of the groom and a wedding three weeks away, I waited as long as I could for the MOB to purchase a dress. I finally purchased one even though she still hasn't. My dress is full length. Now I am hearing that the MOB wants to wear knee length. Should I return mine and start over or is it acceptable for us to wear different lengths?
Both of you should have coordinated with the bridesmaids and the formality of the wedding. Typically, full length is reserved for the most formal of weddings, although the formality of the dress is based on more than just the length.
Based on the fact that you waited so long for the MOB to find a dress, if you feel that your dress is the correct formality and the bridesmaids are all wearing long dresses, it would be appropriate to wear one as well. However, it this is not the case, it is best to look for one that will coordinate better with the MOB.