DNA testing

I'm 27 and I was told risk for downs is 1:844 & after blood work was told it was changed to 1:154 & need a DNA test. The doctor said everything looked perfect in the ultrasound... They also mentioned that there is a 99% chance it is healthy at 1:844 & 1:154, I don't understand that? Has anyone else been through this or had a DNA test & what were the results? Thanks ladies!!