When Gretchen Piscotty was pregnant with her first child, Stephen, she liked to wind up a music-box piano and play “Amazing Grace” for him.

“She thought I could hear it,” the new Oakland A’s outfielder said. “Amazingly, that’s one of my favorite songs. I’ve always loved it.”

In May, Gretchen Piscotty was diagnosed with amyotrophic lateral sclerosis, a neuromuscular disorder also known as Lou Gehrig’s disease. The illness is progressing far more quickly than doctors expected, and Gretchen has lost much of her mobility. Speaking is becoming difficult.

So now it’s Stephen’s turn. He plays “Amazing Grace” for his mother on his guitar and it soothes both of them.

“Whenever I play for her at night, I’ll end with that song and it gives me chills, the good kind, when I play it, as well as a calming peace,” he said. “It’s just crazy the full-circle thoughts and emotions I get when I hear or play that song.”

Giants minor-leaguer Will LaMarche visited the Piscotty home in Pleasanton recently and he and Stephen played their guitars for Gretchen.

“Probably the last 10 minutes was the most beautiful thing I’ve ever seen,” said LaMarche, who played with Stephen at Amador Valley High School. “I’m tearing up thinking about it. Gretchen put her iPad down and listened to Stephen take the lead, and I could sense the mother-son connection there, I just could feel that bond.”

Piscotty is back in the Bay Area full time and living at home after playing for the St. Louis Cardinals for three years. The Cardinals, who drafted Piscotty out of Stanford in 2012, traded him to Oakland in December — a deal that helped both teams from a business standpoint, but also allowed Piscotty to spend more time with his ailing mother.

The Piscotty home always has been a spot for friends and neighbors to congregate. Everyone in town seems to know dad Mike, who works at Lawrence Livermore National Lab; Gretchen, who worked at Foothill High School, and the Piscotty brothers, Stephen, 27, Nick, 24, and Austin, 21, who all played baseball at Amador Valley.

“A lot of people have come around to help, it’s a testament to how they’re seen in Pleasanton,” said Matt Hoey, who also played at Amador Valley with Stephen.

What most impresses visitors is how ordinary the vibe is at the Piscotty place.

“I hung out like I always did and it was like back to normal,” Hoey said. “There are day-to-day things to deal with but the mood is still the same.”

As many as 30,000 Americans are affected by ALS, with 5,000 new cases diagnosed each year, according to Johns Hopkins Medicine. Scientists have been unable to pinpoint a cause for ALS, and there is no known cure, according to the National Institute of Neurological Disorders and Stroke.

“It’s really tough what they’re dealing with but they’re handling it so well, they’re prepared,” said Don Johns, who coached Piscotty’s Connie Mack League team. “It’s a tough disease. Gretchen is a trouper. When I went to see her, she was smiling and positive. Her attitude is so upbeat; she wants no pity. She won’t let the boys feel sorry for her.”

Gretchen, 55, is as funny as ever — she jokes that the time she was proudest of her Stanford graduate son was when he was potty trained. And when Stephen is asked if he has to do chores now that he’s back living at home, she quickly answers for him despite the effort speaking takes. “Yes, he does!” she said with a laugh.

Gretchen was a high school volleyball player and she remained active, regularly working out at boot-camp classes that required her to flip tires. So when she began to have back and foot issues last year, she assumed that she’d just overdone it and that maybe a disk was pressing on a nerve. When the foot problem persisted, though, she got an exam.

“My foot wasn’t flush and my doctor did an MRI, that’s where it started. They couldn’t find a reason, so we ended up at UCSF ... and there you go,” she said of receiving the diagnosis. “Your jaw drops.”

Stephen, then in Los Angeles for a Cardinals series against the Dodgers, was shaken by the news. “We knew her foot wasn’t well and she was limping,” he said. “But you never expect the worst. We thought she’d have back surgery or something and would be fine. I remember getting the news and not wanting to believe it, or thinking that they were just saying it as a precautionary diagnosis. It became more real just hearing my mom’s voice on the phone; I could tell she was a little scared.”

Piscotty took several days away from the Cardinals at the end of May, and the family regrouped in Pleasanton. One early plus: Gretchen was the second patient in the country approved for a new ALS drug, Radicava, which is believed to slow the progression of the disease in some patients.

It did little to slow things for Gretchen, however, and much as she fought it, she wound up in a wheelchair in order to go see Stephen play in St. Louis last year. As the illness has advanced, she now uses a ventilator at times to help with her breathing.

“I think I must have had (ALS) a longer time than I thought,” Gretchen said. “It was really slow, but not on the back end.”

Mike Piscotty, 55, did a lot of research about how to prepare for the effects of ALS, and had the bathroom remodeled and acquired every bit of equipment needed — including an RV with a lift that he bought in Kansas City, Mo., and drove home. Stephen likes to play chauffeur for his mom and her friends.

“We’ve had some fun times in the RV, driving it around, like a taxi service,” Stephen said. “The store, movies, shopping, the spa. It’s actually really good, because I drop them off and it’s a motor home, so you can roam around, put your feet up, take a nap.”

The family attended Thursday’s Warriors’ game, and they took a cruise to Hawaii in December, refusing to let ALS get in the way of good times.

Gretchen plans to attend A’s games at the Coliseum whenever possible. Their family and friends are delighted that Piscotty will be living at home when the team isn’t on the road, and as a bonus, the Piscotty clan are longtime A’s fans. Mike has had season tickets for more than 20 years; Austin even rooted for Oakland against St. Louis. “I just rooted for one guy on the Cardinals,” he said.

For Gretchen, A’s games used to mean me-time while everyone else was at the Coliseum. As Stephen pointed out, even the Piscotty cats, including current resident Cooper (for Cooperstown), have been male.

“I had the house to myself,” she said with a smile. “I had a quiet evening when Mike took the boys to the games.”

Also living back home is Nick, who is working at a startup in San Mateo after pitching for Duke and earning his degree there. Austin is nearby, too, playing for St. Mary’s.

“With or without the illness, I’m very grateful they’re all close by,” Gretchen said. “I wish this wasn’t impacting them, impacting the family, but I’m very grateful to see them. They’re all very tender and caring and gentle with me.”

She and Stephen always have been especially close. “It’s funny, it’s like what you see on TV, like, ‘You’re a hawk with the first kid.’ On occasion, she calls me ‘The experiment,’” Stephen said. “We’ve always had that special bond, because she was always watching me so closely. That strengthened our relationship. It’s hard to describe, but just by raising me, she gave me everything.

“She’s so balanced and so fun and pokes fun at us, and she reminds us there is more to life than baseball.”

And on those quiet nights when nothing more needs to be said, Stephen can bring out his guitar — it’s always close to hand, he said — and play the song that connected them even before he was born.

Tis grace that brought me safe thus far.

And grace will lead me home.

Susan Slusser is a San Francisco Chronicle staff writer. Email: sslusser@sfchronicle.com

Twitter: @susanslusser