What Is Causing My Recurring Miscarriages?

Im 45yrs old woman and im pregnant 3 times but every times when im pregnant im allways miscarried .My question is why im misscaried allways? I have allready 3 children ,me and my husband wants to having more babies ,im so tired to cry and pray hard but is allways lose my baby every two months during my pregnancy ,the babies never develope? or maybe im too old ? Pls, doctor help me and give me a good advice. Can you help me what vitamins or medecine i need ?