Resorting to birth control not to have a deformed child

Q: I am a Muslim surgeon and I have four children. The last child was born nearly two months ago. Right after his birth, he became very sick, and he is now in hospital. But all praise be to Allah, he is getting better. (Part No. 19; Page No. 303) Doctors have found out that the reason for his sickness is natural milk; and this is due to a genetic factor in my wife and me. Although the child is getting better, he must drink a special kind of milk; any type of meat, eggs, or cheese is not allowed. He can only eat some fruits and vegetables. However, by Allah's Support, he can grow up naturally, In sha'a-Allah (if Allah wills). The reason I am writing my letter is that if my wife thinks of having another child, but there is a twenty-five percent probability that the baby will be born with the same disease. This is why we would like to enquire about the ruling if I surgically tie the spermatic duct or my wife undergoes a tubal ligation surgery in order to prevent pregnancy permanently, since Allah (Exalted be He) has already bestowed upon us four children, and we want to avoid having another child like the last one. This is not just for our sake but for the baby's sake as well, as they will have to live all their life suffering this strict diet that no child can bear. Please provide us with a Fatwa regarding the ruling on undergoing a spermatic duct ligation surgery or tubal ligation surgery.

A: This decision should not be based on probabilities; pregnancy and whatever affects the embryo in its mother's womb or after its birth belongs to the realm of the Ghayb (the Unseen) that no one knows but Allah. All the affairs of people are in the Hands of Allah (Exalted be He); He manages them in the way He sees fit. (Part No. 19; Page No. 304) Therefore, you should anticipate goodness from Allah and think positively of Him, as He is as His servant thinks of Him. You should put aside other thoughts and probabilities, and do not be pessimistically driven to use the contraceptive methods you mentioned because of what has befallen your fourth child, for the Grace of Allah (Exalted be He) is immense. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.