Three scans demonstrating the very early progression of ultrasound appearances.  

The very short follow-up period between the first and second scan was probably prompted by some uncertainty as to whether the tiny cyst in the endometrial cavity represented a gestational sac (ie, the case of treated initially as a pregnancy of unknown location). 

Note however that its location is typical for a gestational sac, demonstrating the intradecidual sign. 

Furthermore, Doubilet has previously shown that if a nonspecific intrauterine fluid collection is seen in a woman with a positive pregnancy test, the probability of it being a gestational sac is more than 99.5%¹.  Therefore, provided the patient remained clinically well and with the reassurance of appropriate early bHCG rise, it would have been appropriate to delay the repeat scan to at least 14 days later.

Note that in this case, the scan performed on day 13 still does not prove viability, despite the presence of a very early embryo.  

Patient went on to a normal pregnancy and term delivery.