Spontaneous abortion after established sonographic viability is rare. This study prospectively evaluated 39 late first-trimester abortions after fetal viability was established by ultrasound. The average crown-rump length measurement and mean sac diameter in this group were determined and compared with predicted values based on known conception dates, and the discrepancy between actual and predicted measurements was noted. The same calculations were then made on 39 prospectively matched non-aborting controls. We also determined the difference between sac size and crown-rump length in both groups. Significant differences were found for all three measures. Small-for-dates sac size and small-for-dates crown-rump length were defined as having a discrepancy score greater than 1 standard deviation (SD) above normal. Small-for-dates sac/crown-rump length was defined as having a discrepancy score less than 1 SD below normal. A high percentage of aborters was correctly predicted by a mean sac diameter (82%) or crown-rump length (77%) that was greater than +1 SD, with few false positives (12.8 and 5.1%, respectively). The cutoff for differences of sac minus crown-rump length measurements (10.1 mm) was less sensitive (56%), but just as specific (82%). When both the mean sac diameter and crown-rump length were smaller than expected (according to known date of ovulation), the ability to predict abortion was 71% with few false positives (3.5%).