I suffered a miscarriage of my first baby. I was about 11 or 12 weeks along. There is nothing that you can say. But you can just be there. Be there as you normally would and a little more. If they want to talk about their loss, let them and listen. Say you're sorry, that you're sympathetic with their loss. DON'T say they can have another one. They already know that. They are mourning the loss of THIS baby.
From my experience, I no longer feel grief. It ended with the successful pregnancy of my first born and I had two others besides. Sure, I miscarried a baby. That baby is in heaven and there is nothing I could have done to save the life and I didn't cause the death. I can rest in the assurance that for some reason I don't know, my miscarriage was part of God's sovereign plan. I pray your friends, in due season will be able to reach that assurance as well.
I am sorry for your loss, but you have made a statement that will help my friends and many of others.
I thank you so much and God bless.