Lexington - Zenyatta, the 2010 Horse of the Year, is in foal after being mated with 2006 Preakness winner Bernardini and is expected to give birth next February, her manager announced on Thursday.
VIDEO: Zenyatta wins the 2009 Breeders' Cup
The announcement comes only three days after Rachel Alexandra, the 2009 Horse of the Year, was revealed to be in foal and also expecting next February after being mated two weeks ago with 2007 and 2008 Horse of the Year Curlin.
The pregnancies set the stage for a sensational American Triple Crown series rivalry in 2015, when top three-year-old horses race for glory at the Kentucky Derby, Preakness and Belmont Stakes.
No horse has swept all three races since Affirmed in 1978, who was the third horse in six years to achieve the feat after Secretariat in 1973 and Seattle Slew in 1977.
Before that, no horse had won the treble since Citation in 1948.
Zenyatta retired in November after a 19-1 career, losing only to Blame in the final race of her career in last year's Breeders' Cup Classic.
She set a North American earnings record for female horses with more than $7.3 million.