The 26-year-old Transformers star has been married for two years to Beverly Hills 90210 star Brian Austin Green.
And although the couple has yet to make a formal announcement, friends of the brunette beauty says it's something she has wanted for a long time.
"They're thrilled about the baby!" an insider said. "The pregnancy was planned. She has wanted a baby with Brian for a while."
Megan said earlier this year that she "always loved kids" and adores being a stepmother to Brian's 10-year-old son Kassius from his relationship with his ex Vanessa Marcil.
"Brian knows that Megan is mom material," the insider added. "She is amazing with Kassius."