Preston and her husband, actor John Travolta, are also the parents of a 10-year-old daughter named Ella. Their 16-year-old son, Jett, died following a seizure in January 2009.
Preston's representative Wednesday confirmed the gender of their youngest child to People.com.
The couple was given a baby shower earlier this month, the Web site said.
The child's anticipated due date has not been reported.
Notable deaths of 2014 [PHOTOS]