See below, the website I got it from is listed at the bottom. I know this is true, I studied development for years in graduate school. But quick answer is YES, it's what is expected (your doctor would have told you if it wasn't). Congratulations!!!
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.
from:http://www.americanpregnancy.org/pregnancycomplications/e...