Even very early ultrasound has a margin of error for dating – I think +/- about 5 days. By the third trimester, the error margin increases to +/- 3 weeks!
Having the clomid (and therefore, the ovulation) dates is actually more accurate than using the LMP (last menstrual period). The tradition of using the LMP came about because it’s a piece of objective data, if the woman tracks her cycles. Even then, we have to take her usual cycle length into account. Assigning a due date using the LMP is based on the presumption of starting a period every 28 days, and most women’s natural body rhythms are not that rigid. And I know from years of experience that when I actually ask women who give me an LMP date “how far off might you be with this date?”, less than half are completely certain. Most women will tell me they might be off by a few days, and some are kind of guessing, and might be off by a few weeks! So knowing when you ovulated is more reliable than knowing when you had your last period.