The `Transformers` actress welcomed her first child, a boy, with husband Brian Austin Green in late September, according to E! news.
The 26-year-old star and her husband - whom she married in Hawaii in 2010 - have kept the happy news to themselves since then.
Megan has been similarly private throughout her pregnancy, ending one interview when asked indirectly if she was to have a baby. She only later confirmed she was pregnant by posing for photographers with her bump.
Megan said earlier this year she has "always been maternal" and wants "at least two, probably three kids."
The new arrival will be a brother to Brian`s son Kassius, 10, from a previous relationship and his wife has previously said how being a stepmother makes her extremely happy.
She said: "A lot of things make me happy. But I feel unbelievably happy when my stepson tells me I`m beautiful. When I started living with Brian, his son was only two and I was 18. I`ve been looking after Kassius since he was three - it`s part of my world that very few people really ever understood about me.
"I love being a stepmom and the experience grounds me when nothing makes sense in my life."