Whereabouts do you live?
Most gyms have a 'pay as you go' system, where you can pay as a one-off entry fee. But i suppose it depends on where you are located. I know some 'local' pools which have gyms attached to them are 'pay as you go' pretty much.
I'd love to join the gym now, with bub now 10wks, but what do you do with bub when you go, plus like you mentioned, most really good gyms have expensive Membership fees etc when (knowing me) i'd stop going after a few months (... or weeks!).