Low HCG at 17 weeks? please if anyone has any advice!

So I had my maternal serum screening and we're waiting for our ultrasound next week because some of the results came back abnormal unfortunately. I hadn't seen my chart until I went online but it seems to says my HCG serum is 51.9 IU/mL? Isn't that abnormally low for 17 weeks?! I'll call my doctor tomorrow but my mind is racing, it seems like my whole life is at a stand still. I haven't had any bleeding and feel LO moving pretty regularly. Anyone else experienced this?