U/s are performed to show how far along u r... How many babies u r carrying snd to find out any problems in early pregnancy... I.e before 12weeks. Conception can be different from ur ovulation.. Such as if u hve 28days cycle u ovulate on CD14 thn ur scan should be a nearest match to LMP. Which means if u r 5weeks from ur LMP thn ur scan should show nearly 4 weeks few days or exact 5 weeks... However ovulation can happen at any time dependibg on length of cycle which also effects ur scan... :-). Plus once u get a scan done u dont actually need ur menstrual cycle date because thn actual due date is dertermined from that scan...
THE ULTRASOUND WILL TELL YOU HOW FAR ALONG YOU ARE AND WITH THAT YOU CAN ADD WHEN YOU CONCEIVED. EXAMPLE IF THEY SAY THAT YOUR 5 WEEKS THEN YOU KNOW TO COUNT BACK 5 WEEKS AND THATS WHEN YOU CONCEIVED, ONLY U KNOW WHAT DAYS YOU HAD SEX LIKE THEY WONT BE ABLE TO POINT OUT 1 CERTAIN DATE, THEY COULD PICK A DATE AND YOU WOULDNT EVEN HAD SEX THAT DAY, HOPE THIS HELPS