If there is any question as to whether or not a woman is having a miscarriage based on an ultrasound scan, the usual course of action is that the doctor will order another ultrasound in a few days or a week.
Sometimes, however, the presence of other diagnostic information can help a doctor to interpret ultrasound results even if there is only one ultrasound scan. For example, if a woman has a positive pregnancy test the day of her missed period followed by confirmation with hCG blood test results and an ultrasound four weeks later shows a pregnancy at only five weeks gestational age, the doctor may conclude that the woman has had a missed miscarriage -- because the positive pregnancy test a month earlier would indicate that the pregnancy should be more developed.
In addition, if the ultrasound measurements indicate a fetal pole and/or heartbeat should be present and it is not, the doctor may diagnose miscarriage. The American Pregnancy Association cites the guideline that if the gestational sac is larger than 16-18 millimeters and contains no fetal pole or if the fetal pole is larger than 5 millimeters and has no heartbeat, miscarriage has occurred. (These situations would be called a blighted ovum or missed miscarriage, respectively.)
