Untapable polished off an impressive year with a 1-1/4-length victory in the Breeders' Cup Distaff, with a pregnant Rosie Napravnik becoming the first female jockey to win the race in its 31-year history.

Napravnik stunned the crowd of 37,205 at Santa Anita and a national television audience by announcing she would retire after the second day of Breeders' Cup racing.

The 26-year-old jockey said she and her trainer husband Joe Sharp were starting a family. Later, she confirmed she was seven weeks pregnant.

"His career is brand new and thriving," Napravnik said. "It's kind of good timing. He's going to step into the limelight, and I'm going to step out."

Napravnik has four mounts from the nine Breeders' Cup races on Saturday (Sunday AEDT).

Napravnik won her first race in June 2005 and became the first woman to win the Kentucky Oaks with Believe You Can in 2012.

She and Untapable teamed to win the Oaks in May. Napravnik joined Julie Krone as the only female riders to win a Cup race.

Napravnik, who finished second aboard Tapiture in the $1 million Dirt Mile earlier in the day, did not rule out a return to the saddle.

"It's indefinite. I'm not thinking about a comeback in 10 months," she said, adding, "I can't promise to stay off a horse forever."

Napravnik pumped her right hand after crossing the finish line, having secured her second career Breeders' Cup victory. She won the 2012 Juvenile with Shanghai Bobby.

It was Untapable's fourth Grade One win of the year.