What is the ruling regarding a husband remaining with his wife who became pregnant by her lover before marriage, given his prior knowledge of her relationship and his desire to cover for her, and then his discovery of her engaging in sexual acts with him?

The legitimate marriage contract is valid, and the pregnancy is from you. Consider your wife's repentance, remorse, and improved condition. If she is sincere, open a new page with her and keep her in your wedlock. If you do not see sincere repentance from her and cannot forgive her, then divorce her. Your first choice grants you a generous reward, as you help her repent and conceal her secret, in accordance with the Prophet, peace and blessings be upon him, who said: "Whoever relieves a believer of a worldly grief, Allah will relieve him of a grief on the Day of Resurrection. Whoever eases the difficulty of one in hardship, Allah will ease his difficulty in this world and the Hereafter. Whoever conceals the fault of a Muslim, Allah will conceal his fault in this world and the Hereafter. Allah helps His servant as long as the servant helps his brother." Concealment is required for one who is not known for causing harm, not for one who is notorious for it.