i got an ultrasound today and they told me i was 12w3d pregnant. my due date is nov 25, 2011. i counted back 12w3day from today (5/17/11) and it says that i would have got pregnent feb 18, but when i did the conception calculator online it said my conception date was mar 4. what day did i actually get pregnant? if it was the 4th then why did they start counting weeks the 18th?
The scan measures a gestational age which start counting 2 weeks before the conception date. So when your scan showed 12w3d, the fetal age was only 10w3d. That is why your calendar and calculator gave different dates.
Add to that that your conception date is often not the intercourse date.
So with your dates: you conceived around March 4. Intercourse from around Feb 27 until about March 5 coils have made you pregnant. At 12 weeks, these dates can be off by as much as 4 or 5 days either way. If your earlier scans gave the same date, it is more accurate than the plus/minus 4 or 5 days.