Everyone who's attending Jennifer Lopez's concert at New York City's Madison Square Garden tomorrow night is in for a big surprise -- well, sorta. She's gonna "announce that she and (husband) Marc Anthony are expecting their first child" (otherwise known as the worst kept secret in Hollywood), an insider tells Page Six.

Spies at her Wednesday night show in Connecticut were on baby bump watch, and they had much to report. "A fan blew the side of her top all the way up for a moment, and a definitive baby bump was there. She was very quick to push down her top," the source says. "Then she talked about how this year is full of firsts for her...and every so often her dress would fly up and you would see her belly." Now, there's one way to publicly announce your pregnancy!