Empty canine uterus on ultrasound
The following images are from a canine pregnancy scan of a pug in Addington, Croydon, mated 35 days ago.
Both horns of the uterus were seen, both still fluid-filled, though no foetuses were identified. Given the length of time which has passed since her season (over a month ago), this suggests that the mating was successful, but the pregnancy reabsorbed.
The fact that the fluid is clear (seen in black) suggests that this is a normal and healthy process. Cloudy or speckley fluid would be a worrying sign, and warrant immediate referral to a vet (the bitch may need antibiotics or even surgery for pyometra). Reabsorption early on in the pregnancy, before bone ossification at 40 days, is common and not generally anything to worry about unless, of course, it is persistent problem.