The gestational sac is often the first thing most ultrasounds can detect at the five-week mark of a pregnancy. According to the American Pregnancy Association, it is completely normal to only see a small sac with nothing in it at between the fourth and fifth weeks.
The small sac is seen before a recognizable embryo can be observed. While it appears empty, there actually is a life forming. Around the middle of the fifth week to the beginning of the sixth week, a yolk sac can be seen inside the gestational sac, according to the American Pregnancy Association. It will be the earliest sign of nutrients for the developing fetus.