At 4 weeks 1 day my level was at 34 and then 84 two days later, so 104 is not too low! They will not typically see anything on a u/s until that level reaches about 2000 though.
The ultrasound won't show anything except a thickened uterine lining. You won't see a baby or even a sac with numbers that low. They would need to be in the thousands.
The hCG has to begin somewhere. A late ovulation or a late implantation can result in a primary hCG draw to appear low. But the point is that the numbers need to increase. Doctors like to see them double every 48-72 hours. My first hCG draw this pregnancy was 109 and later we got a sonogram at 8 weeks 5 days pregnant we saw our little gummy bear with a heartbeat.
Just to warn you, you most likely will not see anything on the ultrasound. I'm surprised that they would even perform one this early. Usually you can't see a sac until 5 weeks at the earliest and a heartbeat until 6 weeks at the earliest. Most people don't even see anything until closer to 8-9 weeks via ultrasound.
Thank you for your reply. That really eases my mind. I have been stressed out for three days. They are doing an ultrasound tomorrow. I'm kinda confused about all the tests so early.