How we found out Myah would most likely have Down Syndrome

When we were 12 weeks pregnant we had an ultrasound done. The doctor came in and also gave me an ultrasound which i found weird because doctors don't normally do ultrasounds. Once he was finished he said that baby showed some markers of having Down Syndrome and that i should get tested for it. We had already decided prior to this day that we did not want any testing done we would love our baby regardless. Well we were scared so we agreed to have a blood test done. I got a phone call about a week later from a nurse who worked at the hospital. She informed me that my blood test came back that my baby had a 100% chance of having Down Syndrome. I was so confused, how could just a simple blood test tell you this. The nurse had asked me four times, yes four times if i wanted to terminate my pregnancy. Of course i said no. She asked me if i wanted to get an amniocentesis done i said no but that we would need to talk about it. We opted out on having the amniocentesis done because of the miscarriage risk's. We had another ultrasound done at 20 weeks and they measured her and told us that she had an extra flap of skin on the back of the neck also that she had short humorous bones, she also had an absent nasal bone. That with these signs and the blood test that it was more likely that our baby would most likely have Down Syndrome. After hearing all of this we went home and did some research.