In certain communities it is customary for women to also wash Mayim Achronim, while in other communities it is less prevalent for women to do so. Both Minhagim are acceptable. (See Aruch HaShulchan 181:5, Shu”t Teshuvos V’hanhagos Vol. 1 Siman 174, Shu”t Shevet Haleivi Vol. 4Siman 23 and Shu”t V’Yevarech Dovid Siman 30)
If one did not wash Mayim Achronim and recited Birchas HaMazon, he should wash after completing Birchas HaMazon, or even in the middle of Birchas Hamazon if he remembers, as it’s not a Hefsek as it is necessary for the Bracha.(Mishna Berura 181:2, Sefer Otzar haMayim Chapter 4:52 quoting Rav Chaim Kanievsky Shlita)
[from the wonderful Ben Olam Haba Web site]