This is such a difficult question that I am sure many have asked, especially when one is closely related to or involved with the child with the defect.  I must confess even now, that I don’t believe I can answer your question completely.

Let us consider what we read in John 9:1-7, “And as Jesus passed by, he saw a man which was BLIND FROM HIS BIRTH. And his disciples asked him, saying, Master, who did sin, this man, or his parents, that he was born blind? Jesus answered, Neither hath this man sinned, nor his parents: but that the works of God should be made manifest in him. I must work the works of him that sent me, while it is day: the night cometh, when no man can work. As long as I am in the world, I am the light of the world. When he had thus spoken, he spat on the ground, and made clay of the spittle, and he anointed the eyes of the blind man with the clay, And said unto him, Go, wash in the pool of Siloam, (which is by interpretation, Sent.) He went his way therefore, and washed, and came seeing.”

In seeing this man who was blind from birth, the Lord’s disciples concluded that some sin must have caused his blindness.  Perhaps it was because of some sins his parents committed.  Perhaps it was the man’s own fault that he was born blind.  The Lord assured them that this blindness was not a consequence of sin, but that “the works of God should be made manifest in him.”  In this case, this man was born blind so that the Lord could miraculously heal him and show the power of God to all and show that the Lord Jesus had come to save!

There are many birth defects and not all of them are obvious at birth.  Some defects can be things such as ADD, ADHD, heart defects, etc.  There are also genetic disorders that a parent might pass to their child…things such as depression which might not manifest itself for years or decades.

Sometimes birth defects are the result of the parent’s actions.  If a pregnant woman drinks too much alcohol or takes drugs, or smokes cigarettes, the risk of certain birth defects is much higher.  If a man should hit his pregnant wife or girlfriend, the risk of birth defects is higher. 

Of course, the underlying reason for birth defects is sin!  When Adam and Eve sinned in Genesis 3, we see the consequence of sin entered the world…things such as sickness, disease, and death.  We are all subject to these consequences because we all have Adam and Eve as our parents.  None of us are sheltered from these defects because we are all sinners.  King David remarked in Psalm 51:5, “Behold, I was shapen in iniquity; and in sin did my mother conceive me.”  We all share that beginning with David.

While we may wonder about such things, we need to be so careful that this lack of understanding does not take us away from the Lord and cause us to doubt His love or His power.  Although Job was caused to endure great sufferings, he firmly stated in Job 13:15, “Though he slay me, yet will I trust in him: but I will maintain mine own ways before him.”  Although he could not understand God’s ways (read Romans 11:33), he remained faithful to the Lord.  We may not know why the Lord causes or allows some things to happen, but we can trust in His perfect love and His perfect will. 

The Apostle Paul, although he did not have a birth defect, did receive from the Lord a “thorn in the flesh” (2 Corinthians 12:7).  From this unnamed defect, Paul learned a very valuable lesson.  The Lord told him in 2 Corinthians 12:9, “And he said unto me, My grace is sufficient for thee: for my strength is made perfect in weakness. Most gladly therefore will I rather glory in my infirmities, that the power of Christ may rest upon me.”  Paul learned to be thankful for this defect for his weakness gave room for the power of God to be upon him.  (CC)  (492.6)