Do ultrasounds add two weeks & should they theoretically date back to your lmp?on 12/6/13 baby measured 12w4d, crl was 60.9mm.if go back 12w4d should that date to(or close to) my lmp?when was conceptn?

Gestational dates: Are based from your lmp. If the average female has a 28 day cycle, your ovulation is about day 14. But that could vary by 2-4 days easily. If you only had sex once during your cycle and you got pregnant, you'd know for sure when you conceived. But if you had sex a couple times or more during your mid cycle, that makes it much harder to pinpoint the day.

Yes: Ultrasounds do add 2 weeks, as you suggest, because pregnancy dating is based on your lmp, not your conception date. If on 12/6/13, your baby was measuring 12w and 4 days, then your 'average' date of conception is +/- (5 days on either side) of september 23. Hope that helps.