Measuring 1 week behind

ana

So today I went for an elective ultrasound to get nice 4D pics. I got concerned because she said the head measured 2 weeks behind, but was not a concern because they take the average to calculate the final measurement. So the average came that i was 27 weeks and 3 days instead of 28 weeks and 3 days. She said up to 2 weeks behind is fine, but can’t help but worry. Anyone with similar experience?