Help ?? Telling new boss I’m expecting

So I’m 6 months pregnant.. but I don’t look it at all. I just recently went for a job interview this earlier this week and this morning, I got a call back.

I’ve been fortunate enough to not have any complications or restrictions since I’ve been pregnant and I’m pretty sure it won’t stop me from doing my job.

When should I tell my new boss that I’m expecting ? And how should I do it? I’m scared that they may take the offer back.. but I really need this job, I have to provide for my little one.

FYI: I didn’t know I was pregnant until about a month or ago.. I’m kinda chubby so my belly didn’t look out of the ordinary or anything and I have an irregular cycle that skips periods.