Understanding the ultrasound

When I went in for an ultrasound, we saw a baby in a black space. Is the black space the uterus or the sac? But in twin pregnancies, there are two black spaces so makes me think it is the sac. So if it is the sac, which part of the ultrasound is the uterus? The write colour around the black space that is around the baby? My gyn said baby is in the uterus and I imagined the black space was the uterus. But now come to think of it, at 6 weeks, baby is so small. But the ultrasound does show as if baby is filling almost half of the black space. The uterus can't be that tiny can it.

Felt I dont need to post a photo as there are many such photos already being posted every day here. Hope my questions are not too silly.

Update. Here is my ultrasound