I can confirm that the folllowing stuff from Wikipedia is acurate, because I may discuss this aspect of the book on my blog and peeked at a few appropriate pages to make sure:
Seeking to break out of their suburban rut, April convinces Frank they should move to Paris, where she will work and support him while he realizes his vague ambition to be something other than an office worker. Unfortunately, Frank (from whose point of view most of the novel is told) is a weak reed, doing the minimum to get by at work without developing any alternative self, in contrast with April's taking concrete steps to accomplish their move. When April conceives their third child, their plan to leave America crumbles, not least because Frank is flattered by praise from his supervisors at work and beginning to identify with his mundane job. April realizes that she doesn't know herself any more and that she doesn't love Frank; she tries to abort their child herself, but botches the attempt and dies in her effort to fight the forces keeping her in her suburban housewife lifestyle. Frank grieves, but soon becomes absorbed by the work he had once despised, and "dies" an inward death.
The Wikipedia plot summary for the movie says they were both cheating on each other, as well.
Thanks. One can easily see that it’s all the fault of living in the suburbs instead of in Paris, where there’s never been any adultery or illegal abortions.