What causes MMC?

[Conundrum12345]

I found out today at 9+3 my baby did not grow past 7. We had seen a heartbeat last scan.

What causes MMC? Why don't you bleed or cramp? I'm 35 and worried I won't be able to conceive after

I am so sorry you are going through this.
There are many reasons why, the most common would be a chromosome abnormality. Whatever the reason though it will not be anything you did or didn't do. You wouldn't have had any cramping as your body hasn't recognised the loss yet.
Try not to worry about the future of conceiving just yet, although there is some evidence you are more fertile for 6 months after a miscarriage. Focus on yourself right now and getting through a day at a time.
I am currently going through the exact same thing. Should be 9+1 but at my scan I was showing no more than 6 weeks with my yolk sac collapsing. I just want you to know you are not alone and offer a hand to hold