Sometimes an ultrasound will give uncertain results. For example, if a woman is seven weeks pregnant and the ultrasound does not reveal a fetal heartbeat, the doctor may order another ultrasound in a week. The pregnancy could still be normal but simply off by a few days in the dating -- or the dating could be accurate but the timing is still within the margin of error for when the heartbeat becomes detectable on an ultrasound.
Similarly, if an ultrasound reveals an empty gestational sac, this could still be a normal finding if the pregnancy is early along. The developing baby is too small to be seen on ultrasound until about five weeks of gestation. In this case, the doctor may opt to repeat the ultrasound at a later date.
In either of these cases, the ultrasound results may indicate miscarriage or the pregnancy may still be normal. If the subsequent ultrasound shows that the pregnancy has continued developing, the earlier results can be attributed to problems with dating. If the subsequent ultrasound still shows abnormal development, the doctor can conclusively diagnose a miscarriage -- but the doctor cannot determine the result based only on one early pregnancy ultrasound scan in many cases.
The wait for the repeated ultrasound can be emotionally very difficult, but it may be necessary in order to avoid a misdiagnosis unless other information is present to help the doctor interpret the ultrasound results.