A nonbradycardiac fetal heart rate is associated with a low rate of spontaneous abortion (2%-4%). To determine criteria for predicting impending first-trimester loss when a normal fetal heart rate is identified sonographically, the authors studied 16 consecutively examined patients with pregnancies of 5.5-9 weeks gestation, a small sac size, and fetuses with normal cardiac activity. Mean sac size (MSS) was determined and a small sac was diagnosed when the difference between the MSS and crown-rump length (MSS--CR) was less than 5 mm. Fifty-two consecutively examined patients with pregnancies of 5.5-9 weeks gestation, normal sac size, and fetuses with normal heart rate formed the control group. An MSS--CR of 5 mm or greater was considered normal. Fifteen of the 16 patients (94%) with first-trimester small sacs had spontaneous abortions despite normal sonographic cardiac activity. Four of the 52 control patients (8%) with normal sac sizes had spontaneous abortions. The authors' data show that, despite the presence of fetal cardiac activity at the time of sonography, the usual reassurance provided to patients should be guarded when the sac size is small.