The 'Friends With Kids' star gave birth to her first child Noah with her husband Brian Austin Green in September and the screen beauty admits her maternal feelings are "overwhelming".
Speaking at the March of Dimes Celebration of Babies luncheon in Los Angeles on Friday, Megan said: "The whole thing has been overwhelming because I didn't realise you could love something so much, and I know people always say that but I love him so much it hurts, and it's an interesting feeling to have because I never felt that before."
Megan - who married Brian, 39, in June 2010 - also revealed Noah is currently on the verge of laughing.
The 26-year-old actress is sure her heart will "explode" with joy when he lets out his first giggle.
She added to E! Online: "He's getting to that place where he's smiling and he's almost ready to laugh, and I can't wait for that baby laugh that almost makes you feel like you can fly ... like your heart's going to explode."
Megan recently praised her "soul mate" Brian - who also has a 10-year-old son Kassius, with ex-girlfriend Vanessa Marcil - for being such a great hands-on dad with little Noah.
The Hollywood starlet said: "My husband is amazing. He does more work than I do and he's just a champ ... He's so helpful and nurturing and amazing ... He's my soul mate.
"I love [motherhood]. It's what I've wanted to do since I was a little girl."