Baby size &high risk for Down syndrome

When I was 5 months, I had a blood test revealing that I was high risk for the baby to be Down syndrome. I forgot the number but since then I've been so stressed out. I had my follow up for 5 month ultrasound & midwife revealed that though the baby's anatomy came back normal, baby is smaller than usual. I have 2 healthy babies and this is my 3rd pregnancy. Someone pls help if u have been through this. I was given the option for harmony testing to see if baby does have Down syndrome. I'm 23 weeks rn