Megan and Brian, who already have two sons together - Noah, three, and Bodhi, two - have chosen to name their newest addition Journey River Green.
The Ninja Turtles actress is yet to confirm the news, but is said to have given birth on 4th August.
It was Megan's father Franklin Fox who originally confirmed that the pair were set to welcome another boy into their brood, saying: 'Megan is an amazing mom and I can't believe she is going to have three boys. Becoming a grandfather has been a wonderful experience and brought us all even closer together.
'All I can say is they are together and they are happy. I've met Brian a tonne of times now and he is a great guy... I am the grandfather of her two gorgeous boys and now she has another boy on the way. I couldn't be prouder of the woman my little girl has become.'
Megan started dating Brian when they met on the set of ABC sitcom Hope & Faith in 2004. Although she was 18 at the time and he was 30, Megan has previously stated that 'it was like magic.'
They got engaged in 2006 before marrying in a private ceremony in Hawaii in 2010.
The 30-year-old Transformers star filed for divorce from Brian, 43, last August, after five years of marriage, but it seems the couple have now reconciled and announced they were expecting in April.
A source told UsWeekly: 'They've been through a lot, but this makes more sense for the kids and the new baby,' the insider added.
'Megan is coming around but she hasn't called off the divorce just yet. They recently moved to Malibu together and they've been touring preschools. They're trying to get in as much special time as possible.'
Congratulations to the new mum and dad!