Ritchie, 42, has two sons already from his 8-year marriage to Madonna, Rocco, 10, and David, 5. Rocco was born four months after Madonna and Ritchie got married in 2000 and they adopted David from Malawi, Africa in 2006.
Ainsley, 29, is expected to give birth in September. The couple has been dating for a year.
Ritchie is an English director who rose to fame following the success of his first full-length feature, "Lock, Stock and Two Smoking Barrels" in 1998. He was introduced to Madonna when the soundtrack to his film was released on her label, Maverick Records.
In February, Ritchie's £6 million London mansion was taken over by squatters who ran what they called the "Really Free School," reports the Independent. The squatters claimed to offer classes in "squatting, anarchy and tarot reading" in the mansion. They were booted out later in the month.
Ritchie is currently working on post-production for "Sherlock Holmes: A Game of Shadows" with co-stars Robert Downey Jr. and Jude Law, who presented together at the 83rd Academy Awards last month.