Great Question! I was thinkn maybe in my second trimester to tell them or when im almost 6months, (i should be showing by then)
I think it depends on when you feel comfortable telling them. I just started working at my new job. I'm 22w2d and plan on telling them possibly by the end of the month. I chose not to tell them so soon in fears of getting my application rejected. My boyfriend and I recently moved from NV to CA to be closer to our families and we needed the money. My bump is barely starting to show and I just look a little chubby. But tell the when you feel is a good time :)
In my country you don't have to confirm until you are 30 weeks but it's gonna be pretty obvious at that stage lol. I guess it all depends on if you're full time, part time or casual. Its also illegal in my country to get fired for being pregnant no matter if you're casual or not.
I got a job with my first when I was 18 weeks along. I told them after a couple days just so they knew I wasn't going to be there long term and they were cool with it.
It was only casual work so its not like I took a full term role to try get some maternity leave out of them.