Have you started feeling any movement from your baby? Are you an active person? Have you been using the same scale to weigh yourself? As long as your feeling regular movement baby should be fine, if you are an active person or maybe going thru stress? It could have something to do with it, also if your using didn't scales to weigh yourself it could be off. Although 18 pounds is a lot of weight to loose at this point in your pregnancy. I would definitely start monitoring your weight and also inform your doctor about it, he may want to see you back sooner than you're next appt just to make sure everything is still progressing normally. Unfortunately I can't relate at all, I gained 67lbs with my 1st child and at 31weeks i've already gained 40lbs but at 26weeks you definitely should not be loosing that kind of weight
Maybe your scale is messed up. I don't think that's possible