There's got to be a simple mistake here, and all it would take to straighten it out is to ask your doctor. For example, during your prenatal visits, your health care provider evaluates your fundal height—looking at these measurements certainly would point to gestational age. At 19 weeks, your uterus should come to about your navel, and at 25 it should be about 3-4 inches above it.
Often there are vague spreads of an estimated due date of a week or two, especially if the exact date of the last period is uncertain, but we're talking about six weeks here. This was merely computed wrong and is as simple as correcting this mistake in arithmetic. But you should correct it immediately, because some tests, like the alpha-fetoprotein test, are time dependent, best done between 15 and 20 weeks. The diabetic screen is best done around 26 weeks.
Don't leave your doctor over this—problems like this crop up in the best of practices, and they can be easily corrected.