Is should be 12 size of mentioned class?

Please share your thoughts about this:

 class A
   {
     union {
       unsigned int _dummy_drc[2];
       struct {
         unsigned int _trans_drc :1;
         unsigned int _cap_drc :1;
         unsigned int _fanout_drc :1;
         unsigned int _trans_drc_index :10;
             unsigned int _cap_drc_index :10;
           unsigned int _fanout_drc_index :10;
    //      unsigned int _fanoutk_drc_index :31;
      };
    };
  };

Is it should be 12 and why?