HOT on the heels of JENNIFER ANISTON’s surprise engagement to JUSTIN THEROUX, her mom NANCY DOW dropped a bombshell – she’s going to be a grandmother!

“Nancy said that Jen’s pregnant and that she and Justin would have a wedding soon in Mexico. She’s so beside herself over the news that she couldn’t resist spilling the beans,” said an insider.

“Only a few days after Jen confirmed she and Justin are engaged, Nancy squealed to a few friends and family members, ‘I can’t wait – I’m finally going to be a grandmother!’ ”

Actor/screenwriter Justin proposed to Jen on Aug. 10, his 41st birthday, over a romantic dinner at a New York City restaurant. (read more here)