One of my obs told me that ultrasounds are the least reliable when it comes to estimating a baby's weight. A doctor's estimate is the second most reliable, and a mother's intuition is the most reliable. I have a friend who was tiny during pregnancy but ultrasounds kept saying she'd have a baby over 8 pounds. Her little girl was full term and wasn't even 6.5 pounds. I was way off on my prediction withn my last baby, but she had everyone fooled. Even my doc's prediction was that she'd be about 8.5 pounds. My intuition was off for the whole pregnancy though, this little girl just never gave me any clear signals, which was weird.Of course, if it's your first time it can be hard to figure things out. I wouldn't have known how estimate my first baby's size and weight. All I knew was that she wasn't huge. With my son I could easily tell that he was only a little longer than my daughter and I knew that he would be heavier, but he wasn't going to be a huge baby. (She was 20.5" and 7lbs 15oz, he was 21.5" and 8lbs 9oz.) This last time everyone was off on their estimations. This little girl was 20.5" and 7lbs 5.2oz, my smallest baby. So the fact that she was measuring big for so long didn't mean anything. She wasn't big or long, she was probably just more stretched out. The fact that my belly was enormous didn't mean anything either. Your ultrasound results may be close to your baby's actual size, or they may not be. You'll just have to wait and see. Ultimately, there's no reason to worry. Even if your baby is a little big you'll be fine, and if your sugar levels are under control the chances of your baby having any complications from your gestational diabetes are extremely small.