Week 5 - Gestational Age (Fetal age 3 weeks)
Development
The gestational sac is often the first thing that most transvaginal
ultrasounds can detect at about 5 weeks. This is seen before a recognizable embryo can be seen. Within this week, at about week 5 ½ to the beginning of the 6th week, a yolk sac can be seen inside the gestational sac. The yolk sac will be the earliest source of nutrients for the developing fetus."
So...a gestational sac is all you would expect to see at this stage.