In 12 wk u/s margin of error is 1 wk. Is this error on the due date or error on fetus measurements? Don't fetuses grow at same rate in 1st trim?

Both. When measuring such small dimensions with shadows, the test is going to use a range. When they say 1 week, it is plus or minus a week. As the pregnancy moves further along, the accuracy will improve as they have more baby to measure & features to study.