What Should I Do To Avoid Miscarriages?

hi im 40 years old woman and have an underactive thyroid and ive been takin levo thyroxine for the 7 years. im a mum of 1 and have had problems conceiveing for a number of years ive become pregnant twice in 2010 whilst taking clomid but they ended in miscarriges i was also taking pregnacare conception whilst taking the clomid would this have caused fetal defects to both my pregnancies i am desparate to have another could you please advise what route i should take or if i should take another dose of clomid without taking pregnacare conception.