Claire and Frank (possible spoilers, you've been warned)

Based on information from IMDb forums and Season 3 synoposes

Not actually spoilers, but a theory nonetheless, so half-spoilers.

Rape is one of the reasons Claire might have decided not to have children.

I think Frank is the one in the marriage who is unable to procreate. It's possible Claire might have tried and not had any results from Frank. (I wonder, though, if Frank knows it himself.)

All that would then explain why Claire had had three abortions; because despite having had an otherwise healthy marriage with Frank, she could have been conceived by other men (the day-after pills don't always work), but a different-looking child would eventualy become obvious. And then there's DNA testing.

Claire's biological clock ticking, she decides to get preggers anyway. That's why she wants to separate from Frank and a possible divorce, because she is very certain the child is not his in the first place.

If she's pregnant, then it's possible she might be using it as pretext to disassociate herself from Frank, because she knows that he killed Russo and Zoe. If the murders are eventually revealed as such, she can later claim some plausible deniability. The moodiness might be caused by pregnancy or might even be intentional to throw Frank off the loop, because she needs her female moodiness and the pregancy to disassociate herself from him.