The 26-year-old actress and husband Brian Austin Green - who already has 10-year-old son Kassius with ex-girlfriend Vanessa Marcil - have not confirmed reports that she is pregnant, but friends say they are "beyond thrilled" to have learned they will be having a daughter.
A source said: "She's been helping to raise Brian's 10-year-old son Kassius for years, and now she'll have the experience of being mommy to a little girl.
"Obviously, Megan and Brian would be happy with a healthy baby of either gender, but they're beyond thrilled that Kassius is going to have a little sister."
The couple will welcome their daughter into the world at the end of September with Megan having no movie commitments until 2013.
The source added to America's Star magazine:
"She'll be due at the very end of September or the first week in October.
"Megan is attached to a movie called 'Swindle', but it's in rewrites and probably won't even begin filming until next year. So right now her plate is totally clean, and she can devote herself to her baby."