An intrauterine gestational sac without a visible embryo with a heartbeat constitutes a pregnancy of uncertain viability.  A repeat ultrasound should be performed, in a timeframe whereby the diagnosis of either a viable pregnancy or a miscarriage can be made definitively, as ongoing uncertainty and numerous scans could exacerbate the patient's anxiety. 

Based on the landmark paper by Preisler et al ¹, re-scan is recommended as follows:

MSD <12 mm with no embryo - rescan in 14 days

• if MSD has not doubled and there is still no embryo - diagnosis of failed pregnancy can be made

MSD 12-25 mm with no embryo - rescan in 7 days

• if no embryo with cardiac activity present - diagnosis of failed pregnancy can be made.

Please refer to the relevant article on pregnancy of uncertain viability (PUV) for further diagnosis and follow-up guidelines.   

Note that quantitative bHCG has no role in the diagnosis of pregnancy failure - criteria are based on ultrasound only.  Therefore, the radiology report does need to recommend serial bHCG in the follow-up of PUV.