Hi Doctor, I Had The First Early Naturally Miscarriage Around

Hi Doctor,
I had the first early naturally miscarriage around 8 weeks based on last period. My last menstrual day is June 5. 6 months after, I got pregnant again, and this is my second pregnancy. I took a pregnancy test on July 8 and it showed positive for pregnancy. I have irregular period cycle 28 days - 38 days. My doctor gave me a couple of requisitions one for blood test and one for dating ultrasound. I went for the first blood test on July 24 and confirmed that I'm pregnant, and dating ultrasound on August 11. However, they cannot find the heartbeat. My doctor sending me again to get the 3 blood tests for 3 days, each time within 48 hours. She didn't tell me the exact number, but she said the result showing the HCG have gone down not significantly but it's shows downward, and she advised me either I can wait for the miscarriage to happen naturally or she will refer to for to the medical place where they will more likely give me a pill or schedule D&C for me. I haven't had any spotting, bleeding or cramping. I still feel very tired, and have feeling of morning sickness anytime during the day/night. My questions is if it's for sure I will have a miscarriage? or I still have a chance of hope that the baby will have a heartbeat later due to my irregular period cycle? How long does it take from when they found out my HCG decreasing until I have a miscarriage? I heard there is some point in the pregnancy, the HCG will start dropping when it reaches to the highest lever. I'm pretty confused about my situation. Any advised will help!

Thanks a lot,
Hannah