Fanon: Many people believe that the father of Sonia's child was supposed to be Alucard. While there's no hard proof in-game, the two seem to be close and speak to each other affectionately, and in the best ending Sonia's child is stated to "carry on the fate and tragedy of the Belmont family, and the bloodline of dark ways." As for the dialogue:
Alucard: Please understand. I am the only one who can make amends for the sins committed by my father, and there is no reason for you to get involved in this battle. Sonia, I could not bear to lose you, too. Now, turn back!
Sonia: Alucard. I'm so sorry.
Alucard: Do not trouble yourself about it, Sonia. Now, I must sleep. I fear we shall not meet again. Farewell, my beloved, my beautiful vampire hunter.
Sonia: Alucard! Alucard, I will never forget you. Farewell, dear Alucard.