Why does God allow birth defects?

Thanks for the question. I have attached a useful resource which would help answer the question.
Also, do understand that God has the big picture in mind. Our life is not limited to this earth. It is beyond this world. We have an eternal life. In that place, there will be no health deformities, nor sickness or pain. So as a Christian we do have an eternal hope of a completely restored world. Revelation 21:3,4 says, “And I heard a loud voice from the throne saying, “Look! God’s dwelling place is now among the people, and he will dwell with them. They will be his people, and God himself will be with them and be their God. ‘He will wipe every tear from their eyes. There will be no more death or mourning or crying or pain, for the old order of things has passed away.” With such a wonderful future ahead, a person born here with deformities can still live a life that’s God glorifying and purposeful!