Back ache is your body's way of moving your hips and birthing area around so that when it comes time for the baby to be born and also durring normal growth in the womb things shift to make room and so you are going to continue to have back ache until the birth (most likly) after birth the body will release a hormone called relaxin and all of your connective tissue/tendons will relax and you will actually be very free from pain for a while.
Be prepared to grow at least one full shoe size after birth, your feel will be a part of the wonders of relaxin.
Good luck and dont be afraid to take tylenol for help!