Had a natural missed miscarriage at 12 weeks 2 days, but baby passed at 9 weeks. i'm 39, but healthy. how soon can i start to have baby again?

Immediately: If you don't have any medical issues or uterine factors i would wait for my next period and try again.

I would recommend a : Pt should have a complete hx and physical and then her husband should be evaluated. Women most of the time think they are the ones for a particular problem but their partner may have some contibution. After all is well i recommend pregnancy attempt after 3-6 months.