They usually don't choose to conceive. Another factor is poor planning. Many people don't budget in the fact that one parent could be unemployed for months on end.
I think this deserves extra attention. America has a weird cultural thing at play here; the religious right seem to be strongly anti-contraception (the recent Hobby Lobby decision being a good example). Yet, when the inevitable happens and a child is unexpectedly conceived, there doesn't appear to be much in way of a support structure.
