What problems does being underweight and pregnant cause? ?

I was weighing about 105 before I pregnant. I gained a lot though. However, I have two friends who have also recently had babies, and they also only looked about 3 mos pregnant at even 9 months. One of them even had TWINS and was tiny. If your doctor's not worried, I wouldn't worry about it either. Just be glad you didn't gain 60 lbs! oh my goodness, how it frustrates me. =S lol