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."