Needing advice. Trigger warning possible miscarriage.

I’m currently 4 weeks pregnant. I had a hcg blood test done last Thursday 8/22 to confirm, it came back at 21. My doctor wanted to follow up today Monday 8/26 to do more to make sure it was going up. He said it was in the normal range just low. My levels came back at 17. He said he wants to have an ultrasound done before he tells me a for sure answer but he did mention this pregnancy not being viable. I have been taking at home strips and they have gotten darker everyday since Thursday. I’m just at a loss of words and confused. Has anyone had this happen and have the baby get through It? Or should I start preparing for the worst?