Kirsty, I know the dilemma!! I had applied for a couple of permanent positions for when my contract finished in June and then I found out I was pregnant! With temp positions I have told them so they understand why I was only interested in temp/casual when they asked why and they were fine with it.

My friend also just started a new job not long ago and about 2 weeks after starting found out she was pregnant (approx. 8 wks) and they've been fine as she said she didn't know before. So like Bon said you could take that approach? Or if you feel comfortable in the interview let them know then but that you still want to take the job.