My first child measured 9 pounds at 35 weeks and was born weighing 7 lbs, 13 oz, 10 days early. He was "supposed" to be 10 1/2 pounds at birth. My second measured out at 8 lbs., 6 oz. at 37 weeks and was born a day early weighing 7 lbs., 14 oz. He was supposed to be 10 pounds.
My sister's child measured 7 pounds at their last ultrasound and was born 5 days later, weighing 11 pounds, 6 oz. lol
A friend of mine went in and was told her baby weighed 6 pounds at 40 weeks, then he was born a week later weighing well over 10 pounds!
These 3 measurements were taken by 4 different sonographers, and my sister and friend and I all went to different practices.
I KNOW this MIGHT not help you feel any better, but those scans can be soooooo far off. Another thing to take into consideration is that even if your baby is only almost 5 pounds now, that would probably be an almost-7-pound baby in 4 weeks when you're due, which seems to be around the size of your oldest.
My doctor told me that each baby will receive one growth gene for birth weight, coming from either one parent or the other. I was born weighing 8 lbs 6 oz. My OH was born 3 weeks late weighing 7 lbs., 6 oz. My kids have gotten my growth gene up till this point. Now, my baby girl is measuring much, much smaller than the other 2 and looks like she's gotten my OH's.
How big were you and your OH at birth to compare with your children's sizes?