Yes, I know what causes that
Until recently, when asked that question, I would just laugh at their ignorance. But now I have a different answer.
I know several couples that desire children. Some have struggled with infertility and all the treatments and interventions that go along with that diagnosis. Still others have lost babies to miscarriage and birth defects. These families long for the blessing of children.
In the middle of all this, my wife and I, who already have 5 perfect, healthy, beautiful children, asked the Lord that if it was His will, we would love to have more children. Four weeks ago, our healthy new baby boy was born.
So when people ask me "Don't you know what causes that", I now say "Yes, the Lord has made it painfully clear to me recently what causes that. Nothing more than the underserved loving grace and sovereignty of God causes that."
That answer, along with the above explanation, usually results in one person who never asks that question again.