The news was confirmed by the 'Transformers' star's publicist Leslie Sloane Zelnick in a statement to The New York Post.
"I can confirm Megan is expecting her second child with her husband Brian. They are both very happy," he said.
The couple - who married in 2010 - are already parents to nine-month-old son Noah Shannon.
They kept news of his arrival secret for two months, before announcing it last December.
Megan later posed for a racy Esquire photoshoot in January, revealing she was almost back to her pre-pregnancy weight.
She is currently filming her role in the 'Teenage Mutant Ninja Turtles' reboot, playing April O'Neil.