No, for a number of reasons. A pregnancy is usually dated from the lmp, however you are not likely actually pregnant at that time. A week or two after that would be more of the norm. So while the gestational age, time from lmp, may be 25 weeks, the fertilizational age, from when you actually conceived, may be closer to 23 weeks. The red flag is raised if the weeks are off considerably or if they progress to be farther and farther behind.

Another reason is that even ultrasound is not exact for dating a pregnancy, as babies can have some differences in rate of growth and development. It is considered the most accurate of means for dating however, and this is why sometimes a due date will be altered a bit once an ultrasound is done. Regardless, you will still have several weeks at the end of your pregnancy for what would be considered "term" or a normal time for giving birth.