Though she tried her best to hide her heartbreak, Loki could tell by the subtle clues, such as her balled up fists, that she was upset and he relished the pain he caused with glee. He liked to think he was much more adept at masking his emotions, but when Wanda revealed she was pregnant, that mask slipped, revealing his surprise.
"What?!" he exclaimed, then looked up and over Wanda's shoulder at Thor. As the Lord of Lies, he automatically wondered whether or not she was lying herself, as a way to get back at him. However, Loki knew that his brother was too noble to jade the truth, no matter how many times Loki had been dishonest.
No matter. Loki returned his attention to Wanda with increased savagery. "What makes you think I care about you or the loathsome maggot writhing in your womb?!" His voice rose to a fevered pitch and he leaned forward to drive home his point. "I am LOKI! Do you not know how many slits like you I've ravished over the years?! The amount of children I've sired?! You are nothing to me, mortal! Barely worth my attention. I used you, and now that I am finished you are like the refuse I excrete from my bowels." By the end, his face had distorted into a sneer, horrible to look upon.
And if he continued to think of Wanda in this way, he hoped that eventually the pain stinging in his heart would dissipate. The crescendo of emotion might've been a hint that he actually did care more than he let on.