Here's an important research study that shows what we all know to be true, that ultrasound in late pregnancy is not a reliable source of weight estimation.
http://www.nzma.org.nz/journal/119-1241/2146/Conclusion: The accuracy of ultrasound estimations of fetal weight performed at Wellington Hospital within 7 days of delivery in term singleton pregnancies was at least similar and sometimes better than that reported in other studies. For one in four women, however, the fetal weight estimation was more than 10% different from the actual birth weight of their infant. Ultrasound measurements had a tendency to overestimate the weight of small infants while underestimating the weight of both large infants and the infants of diabetic mothers. As the reliability of ultrasound estimation of fetal weight to detect larger babies was poor, the use of such an objective measurement in the management of suspected macrosomia in term singleton pregnancies should be avoided.