31 weeks pregnant ? pls help underweight baby?

Ultrasounds (esp. those in the third trimester) are horribly inaccurate. I had an ultrasound on a Friday with my second child (I was 38 weeks at the time) and they said he was just over 7 lb. He was born *the very next day* via emergency c-section and he was 8 lb. 14 oz. Don't put too much faith in ultrasound measurements...they can be off in either direction. As long as everything else is okay (baby's heart rate, organs, bones, movement) and your urine is coming out clean, don't worry about it.