What Are The Chances Of Downs In Case Of Normal Pregnancy?

hi i am 35 yrs old i have gyne problem. i had downs child she died at the age of 3. I also had 1 miscarriage. after that i had 3 times week pregnancy and get miscarriage. i want to know about what can i do to be pregnant and test tube baby is suitable for me and how many chances of downs in the case of normal pregnancy and test tube baby .