The accuracy of an ultrasound for determining the gestational age of a fetus depends on several factors, including the skill of the ultrasound technician, the quality of the ultrasound equipment, and the stage of pregnancy. In general, an ultrasound performed at 18 weeks is considered more accurate than one performed at 7 weeks. This is because at 18 weeks, the fetus is larger and more fully developed, allowing for more accurate measurements to be taken. Additionally, at 7 weeks, the gestational age is still relatively early and the fetus is rapidly developing, which can make it more difficult to accurately determine the gestational age.
According to the information you provided, you were 19 weeks and 3 days pregnant when you had an ultrasound. This means that the ultrasound was performed at a time when the fetus was relatively well-developed and the gestational age could be more accurately determined. Therefore, the results of the ultrasound performed at 19 weeks and 3 days are likely to be more accurate than the results of an ultrasound performed at 7 weeks.