We'll need to take up references <a href=" http://fittor.top/ ">unga fittor</a> A sonogram performed in the first trimester is said to be accurate to within a week, when done in the second trimester within two weeks, and within the third trimester within three weeks. Thus, the accuracy of an ultrasound to provide information on the size of the fetus that can project the due date is greatest within the first trimester.