ARCADIA, Calif. — Rosie Napravnik was riding with a secret, and she couldn’t keep it any longer once Untapable won the $2 million Breeders’ Cup Distaff.

Napravnik, the top jockey at the Fair Grounds the past four seasons, stunned a crowd of 37,205 at Santa Anita and a national television audience on Friday by announcing she will retire Sunday. The 26-year-old jockey later said she is seven weeks pregnant.

“I don’t know if I had won on another horse if I would have said anything,” Napravnik said. “This filly has just been very special to me, and it’s a very special way to go out, so I just couldn’t resist because they asked me how much it meant to me.”

Napravnik became the first female jockey to win the Distaff in the race’s 31-year history. She will ride in four of the nine Cup races Saturday before calling it quits. She is married to Joe Sharp, who recently went out on his own as a trainer and has found success in Kentucky.

“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.”

She had planned to tell her mother about her pregnancy at dinner Friday and announce the news publicly after Saturday’s races, but Untapable helped change her mind.

“She just ran excellent today,” said Napravnik, who was aboard for Untapable’s fourth Grade 1 win this year. Napravnik missed the filly’s win in the Mother Goose because of injury.

“Down the backside, I could not have had more confidence.”

Her mother, Cindy, looked on in shock as Napravnik spilled the beans on TV.

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’s best finish in a Triple Crown race was third in the 2013 Preakness with Mylute.

Napravnik, who finished second aboard Tapiture in the $1 million Dirt Mile, didn’t rule out a return to the saddle.

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

Untapable won by 11/4 lengths, running 11?8 miles in 1:48.68 and paying $5.20, $3.40 and $2.60 as the 8-5 favorite in the 11-horse field. Don’t Tell Sophia returned $4.60 and $3.40, while Iotapa was another nose back in third and paid $4 to show.

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 the fifth Cup win for trainer Steve Asmussen and his biggest since winning the $5 million Classic with Curlin in 2007.

“Rosie and she are a great combination, and they have had a tremendous season,” Asmussen said. “She’s an amazing mare to put us on this stage and to perform like she did.”

Asmussen also trains Tapiture, who lost his race by 11/4 lengths.

Untapable lugged in on Iotapa on the rail in the stretch, but the stewards did not post the inquiry sign.

The path to the winner’s circle was made clearer when Beholder didn’t defend her title after she spiked a fever recently.

Trainer Wesley Ward had a victory and two second-place finishes on opening day of the world championships. But he wasn’t present, having stayed home to spend time with his children. He won the $1 million Juvenile Turf with Hootenanny, with his Luck of the Kitten finishing second. Ward also was second with Sunset Glow in the Juvenile Fillies Turf.

Goldencents successfully defended his title in the $1 million Dirt Mile, winning by 11/4 lengths despite tiring in the stretch. He is headed to retirement, where he will stand stud duty at Spendthrift Farm in Kentucky.

Vicar’s In Trouble was vanned off after the race with a minor left front foot injury, according to an on-call veterinarian.

Jockey Irad Ortiz Jr. earned his first Cup victory, guiding Lady Eli over one mile in 1:33.41. She paid $6.80, $4.20 and $3 while improving to 3-0 in her young career.