The couple have been dating since 2010 and are the parents of a 10-month-old son.
UsMagazine.com said Ainsley, 30, showed off her baby bump at the London premiere of "The Dark Knight Rises" Wednesday night.
Ritchie, 43, also has two children with U.S. pop star Madonna, whom he divorced in 2008 after eight years of marriage. Ritchie directed "Lock, Stock and Two Smoking Barrels," "Snatch" and "Sherlock Holmes."
Celebrity Families of 2014 [PHOTOS]