Have they realized that if she hadn't miscarried, they wouldn't be pregnant with this child now? She'd still be pregnant with the first pregnancy - assuming the story that she and Jessa had the same due date was true.
I have a friend who miscarried at 6 weeks, and got pregnant again 10 weeks later. Her daughter is now 13, and my friend frequently says she was so close to never having this amazing girl in her life. If she hadn't miscarried, her daughter wouldn't be here.
I have another friend whose first child died in utero at 30 weeks, which was absolutely devastating to her and her husband. She had to be induced and deliver the baby (and for some reason without an epidural - way to go, Ontario health care), which they named and buried. Ten weeks later she got pregnant again, and carried a healthy son to term. She has said that she and her husband still mourn their lost child, and both of her sons know there was an older sibling who died. She says she struggles with the pain of her loss, and the knowledge that if her first child had survived she wouldn't have her incredible 11-year-old son.
It's complicated. Heartbreak and joy.