Arnold's Baby Mom & Maria Preggo at Same Time

5/18/2011 8:30 AM PDT

Maria Shriver & Mildred Patricia Baena -- Pregnant Together!

Maria Shriver and Mildred Patricia Baena were both pregnant with Arnold's children ... AT THE SAME TIME.

And get this ... the two kids were born less than a WEEK APART  -- Maria's son Christopher was born on September 27, 1997 ... and Mildred's love child was born on October 2, 1997.

The two boys are now 13 years old.