How accurate are first trimester ultrasounds? How accurate are ultrasounds done during the first trimester of pregnancy? I had one done and the doc told me that I'm 8 weeks pregnant, but how off could he be? Is there a week or 2 margin of error?

Small error margin. A first trimester ultrasound may have an error range of 3 to 5 days, but no more than 7. Also, note that dating a pregnancy is done from the first day of the last menstrual period, not from the suspected conception date.
The . The earlier an ultrasound is performed during pregnancy the more accurate it is likely to be. Here's a general rule of thumb: a due date given during the first trimester should be within 1 week of the true due date; during the second trimester it is within 2 weeks; and during the third trimester it is within 3 weeks. An 8 week ultrasound is quite accurate and if it differs more than 1 week from your due date based on your last menstrual period, then your obstetrician will assume the ultrasound to be more accurate. In your case, if you were told that you were 8 weeks along you could have actually been between 7 weeks and 9 weeks and your due date also has a +/- 1 week range of error. You will probably have another ultrasound during your second trimester. The technologist will recalculate a due date; this will have a larger margin of error but should be within 2 weeks of the initial due date you were given (remember the rule of thumb described above). Because the earliest ultrasound is considered the most accurate, the due date given at the first ultrasound is usually not changed by the obstetrician.