other than back ache caused by sleeping on your back, there's another reason why sleeping on the side is good.
babies' backbone is the most heavy part. so when you lie on your back, the baby will have his face facing towards the ceiling with his spine against your spine. this is the worse position the baby can get for labor. my baby was in that position and my doc had to put in extra effort to get him out. nurses have to give extra pushes on my tummy and i have to put in extra effort in pushing. (almost had to go ceasarean because of other complications of cord wrapping baby's neck). it's not the optimum position. the best position is for the baby's spine to face outside. this is extremely important before the weeks leading up to labor. hence, sleeping on your side is best!
since you are going to reach 3rd trimester, as the day draws near, when you watch tv, go on all 4s position or kneel down and lean forward on a sofa so your tummy faces down. this helps baby to get into the optimum position.
hope this helps!
so excited for all of you! reminds me of my pregnancy once again. full of expectations!