When should I tell my boss my wife is pregnant after starting a new job?

My wife and I found out that she is pregnant while I was job searching after being laid off from my last job. At present she is 3 months pregnant and due in July.

I started a new job last week (big step up from my previous one!) And am now wondering when the best time to tell my boss that my wife is expecting? I plan on taking a week off when the baby comes, but of course I'm not going to know which week that is until my wife goes into labor.

What do you all think? TIA!