Either MEGAN FOX is good at keeping secrets, or she's OVER.  Because she and BRIAN AUSTIN GREEN became parents a few weeks ago, and nobody knew it until Megan announced it yesterday on Facebook.

Megan gave birth to a baby boy on September 27th.  His name is relatively normal on the spectrum of celebrity baby names:  Noah Shannon Green.  Megan said, "We have been very lucky to have had a peaceful few weeks at home . . . He is healthy, happy and perfect."  She added,  "We are humbled to have the opportunity to call ourselves the parents of this beautiful soul and I am forever grateful to God for allowing me to know this kind of boundless, immaculate love."

Megan already has some practice being a mom.  Brian has a son named Kassius with his ex VANESSA MARCIL.  He's 10 years old now.  (Brian is 39 . . . Megan is 26.)