Anna Karenina Part 2, Chapters 17-25
Anna realizes she is pregnant with Vronsky's baby, and she informs Vronsky of this. In another climactic event, Vronsky loses a horserace he is slated to win. Vronsky loses the race because he is distracted by his relationship with Anna, especially because his brother and mother very much disapprove of the affair. His mother thinks he is becoming too dangerously preoccupied with a married woman. Further, Vronsky is becoming frustrated at having to hide his life with Anna. With all that on his mind, Vronsky falls off his horse during the race and the horse, Frou Frou, dies:
"She flew over the ditch as though not noticing it. She flew over it like a bird; but at the same instant Vronsky, to his horror, felt that he had failed to keep up with the mare's pace, that he had, he did not know how, made a fearful, unpardonable mistake, in recovering his seat in the saddle. All at once his position had shifted and he knew something awful had happened." Part 2, Chapter 21, pg. 195
Right before the race Vronsky goes to see Anna and learns of the pregnancy. Anna is astounded at Vronsky's reaction--he tells her she should come live with him and leave her husband and son once and for all. He proves that he knows her well when he tells her she is suffering from the guilt of society and her family, and she can never really be a whole person again unless she detaches herself from those forces.
"He vividly recalled all the constantly recurring instances of inevitable necessity for lying and deceit, which were so against his natural bent. He recalled particularly vividly the shame he had more than once detected in her at this necessity for lying and deceit. And he experiences the strange feeling that had sometimes come upon him since his secret love for Anna. This was a feeling of loathing for something--whether for Aleksey Alexandrovich, or for himself, or for the whole world, he could not have said. But he always drove away this strange feeling. Now, too, he shook it off and continued the thread of his thoughts." Part 2, Chapter 25, pg. 211
It is a big moment for Vronsky, one unanticipated. Still, stubborn Anna will not listen to his reasoning. She will proceed in torment.