"You drop some weight just by running after [him]," the star of The International tells PEOPLE

By Elaine Aradillas
Updated February 12, 2009 07:00 PM
Credit: Eamonn McCormack/WireImage

Naomi Watts says she can thank her first child for helping her get back in shape after giving birth to her second.

“You drop some weight just by running after them,” she tells PEOPLE.

Watts, 40, most recently gave birth to son Samuel Kai with boyfriend Liev Schreiber. Their first child, Alexander Pete, was born in July 2007.

The Aussie, who can be seen co-starring with Clive Owen in The International, exercised and walked a lot during her pregnancy. She adds that nursing also helped her drop the weight.

“I was hungry the whole time and I’m still hungry all the time!”