What Leads To Miscarriages?

i am 35 yrs old and having one daughter 6yrs old, before her and after her also 2 miscarrige were happened withing 6 weeks of conception. All test are done as per suggested by my Gynae doctor, everything was normal so requesting to suggest me if this miscarrige could be repeted again or not as I want to concieve again. Thanking you.

Anna