Why do some docs say that the accuracy of conception date determined by trans u/s depend if i have a reg 28 day cycle and some say that it doesnt?

Confusing-ask doctor: Conception does not necessarily occur on the same day you have sex, so the most accurate date is a known last period. A woman's cycles can be 21-35 days apart, and ovulation occurs on day 7-9 on shorter cycles, around day 14-16 for the "classic" 28 day cycle or day 18-21 for the longer cycle. Us dating assumes the average cycle, so a 7 week pregnancy would indicate conception about 5 weeks ago.