She's pregnant so she probably feels like she's locked in and can't do anything about it at this point, even if she wants to leave because she fucking hates her life now. Japan isn't exactly big on abortions. This one instance of cheating with the only person who ever cared about her is meant to be the last moment of happiness she'll feel, before she punishes herself by marrying someone she doesn't give a shit about, and raising a kid she didn't want.
On the contrary, Japan is pretty big on abortions... It's not talked about, but they happen pretty frequently. You have to have a man with you to sign for it (or had to for a long time, not sure if still true), but the usual way around that is to find a friend or something to do it (this was the beginning plot point for a manga I read, I can't remember, the main character gets talked into going to a clinic to sign though it's someone else's kid). For a long time, the pill was not available in Japan either, but it is around now though not that popular or well known compared to the west. So yeah, lots of abortion. Nobody talks about it.
Well, we'll see what happens but I have to agree with other posters, Maki has made her play. She has put it out there, thrown the line, and Midori has to choose to take it or continue doing what she's doing. I mean, if you tell them pretty much, "fuck your fiance, run away with me to a beach town somewhere" you can't be much more assertive than that without crossing over the line into kidnapping territory. That's already stepping all over the "friendship" line, that along with the kiss... This is already pretty far into affair territory, even if they haven't done more than kiss.
This is kinda like a weird reflection of "Like a Spring Storm", both have the marriage thing but one is before and one is after, one has pregnancy and one doesn't, one is fluffy and one is straight angst.