A viable intrauterine pregnancy is a medical term used to refer to what is essentially a healthy, normal pregnancy. "Intrauterine" simply means that the pregnancy is occurring inside the uterus, as it should be, and this can be determined early with a transvaginal or transabdominal ultrasound scan.
Once the pregnancy has been established as intrauterine or "normal," its viability will be determined based on the results of further ultrasound scans.
Medical professionals will first check for a heartbeat within the fetus. If they cannot detect one, they will measure one of two other things, based on whether there is a visible fetal pole. This is defined as a thickening of the yolk sac, the first visible sign of a gestating fetus, and it typically appears at around 6 weeks but sometimes as late as nine.
If there is a visible fetal pole, medical professionals will measure its crown-rump length. They will be looking for a length of more than 7.0 millimeters, but without a visible heartbeat, further scans are typically required to determine the viability of the pregnancy.
In cases where the fetal pole has not yet manifested, the diameter of the gestational sac will be measured. Again, further scans are often required if there is no visible heartbeat.