Is it permissible for a husband to prevent his wife from conceiving if the fetus will carry a hereditary disease that does not affect its health and shows no symptoms, knowing that the reason for the prevention is the husband's fear of future hardship for the children in marriage?

The scholars have explicitly stipulated the prohibition of permanent birth control, except in rare cases. The reason mentioned in the question—the husband carrying a hereditary disease without affecting the health of the fetus—does not justify preventing conception, especially since the disease is not necessarily transmitted to the children. The husband may also find another healthy wife.

It is advisable to rely on Allah and entrust the matter to Him, for everything is by His decree. It is permissible to temporarily postpone conception for a valid reason if both spouses agree to it, as procreation is a right for both of them.