During the fourth week of pregnancy, a small gestational sac forms within the endometrial lining. This can be seen in transvaginal ultrasound pregnancy can be seen in fifth week. Basically in the fourth week, a gestational sac, and a thick endometrium which looks like a little dark blob in the sonogram can be seen in the ultrasound.
When I was pregnant, my doctor took ultrasound of fetus during the first month. But only a sac like feature was seen in the ultrasound image. You need to remember that the baby is not visible in this stage. As the pregnancy progresses, you will be able to see the baby growing in the womb.