When Margaret Abe-Koga was diagnosed with breast cancer in 2015, she was as surprised as anyone.

After all, no one ever had breast cancer in her Japanese-American family, she doesn’t have the genetic marker, and she’d been led to believe that Asian-Americans weren’t — as Abe-Koga put it — “a high-propensity group’’ for the disease.

But fate proved otherwise for the three-term Mountain View city councilwoman, as it has for a growing number of Asian-Americans in California confronting a sobering trend: While breast cancer rates have plateaued or declined in some racial groups, they have been steadily rising among Asian-Americans since 1988.

The new findings, released last week by the Fremont-based Cancer Prevention Institute of California, show the largest increase in breast cancer rates in the Golden State is occurring among Koreans and Southeast Asians. Japanese-Americans showed the slowest increases, but suffered the highest breast cancer rates among seven Asian-American groups in the study.

Related Articles Alameda County has reported a huge COVID-19 death total in recent weeks. Here’s why.

California recalls N95 masks from Santa Clara company with $90 million state contract

UC Berkeley to take part in resurgence of research in psychedelic drugs

Experts say wildfire smoke can make people more susceptible to COVID-19

Coronavirus: With cases way down, California’s decline begins to slow down The results stunned Abe-Koga, 46, who in January went through breast reconstruction surgery after undergoing a double mastectomy, chemotherapy and radiation. For generations, she said, Asian-Americans have been under the wrong impression that breast cancer “is not prevalent in our community” so “it’s not something that people think about.’’

“I started to think maybe there is that aspect within our community: Our folks are more silent about what they are going through and don’t necessarily share, or they aren’t getting the testing they should get,” she said.

The confusion among Asian-American women is understandable. As of 2013, the U.S. Centers for Disease Control and Prevention reported that white women had the highest rate of breast cancer, followed by black, Hispanic, Asian/Pacific Islander and American Indian/Alaska Native women.

But the CDC’s numbers are nationwide. By comparison, the new study is based on the California Cancer Registry in a state that has the largest population of Asian-Americans in the country. With a population of approximately 5.7 million in California, Asian-Americans now make up about 15 percent of the state’s population.

Scarlett Lin Gomez, the study’s lead author, said the results are revealing because they are the first to evaluate patterns among seven major Asian-American ethnic groups, by age and stage of cancer.

It showed breast cancer rates rose among Korean women in California an average of 4.7 percent each year from 1988 to 2006, before slightly declining over the next seven years. The rates climbed an average of 2.5 percent a year from 1998 to 2013 among Southeast Asians (Cambodians, Laotians, Hmong and Thai), and 1.4 percent among South Asians (Indians and Pakistanis).

There is no consensus on why breast cancer rates are on the rise among Asian-Americans. But risk factors could include delaying childbirth, changing diets, a rise in obesity and alcohol use. Better screening could also be a reason behind the increase.

Early studies from the 1970s and 1980s showed increasing breast cancer rates among Japanese-Americans, but they have leveled off. Lin Gomez said that may be because Japanese have been in the U.S. longer than the other Asian ethnic groups whose numbers are growing.

“We know that breast cancer risk increases with acculturation — that is, adoption of Western lifestyles,’’ Lin Gomez said. And that is consistent with the fact that the most rapidly increasing breast cancer rates are seen among the more recently immigrated groups such as Koreans, South Asians, and Southeast Asians.

The study hits close to home for Lin Gomez, who is Chinese-American, and said aspects of the Asian culture could be contributing to the growing numbers, including a tendency to consider cancer a stigma — and to keep quiet within a family.

Grace Yoo, a professor of Asian-American Studies at San Francisco State University who has written extensively on Asian-American health issues, including breast cancer, is familiar with those sentiments.

There is a perception among some Asian-Americans that breast cancer is “a white woman’s disease,” Yoo said. “It’s just not on their radar.’’

Yoo said many Asian-Americans arrive in the U.S. with no family history of the disease “so there isn’t that inter-generational communication’’ around the importance of breast cancer screenings. Diagnosed at later stages, these women then face greater mortality than their white counterparts, she said.

And because many of these women equate the diagnosis as a death sentence, she said, they may not aggressively pursue treatment.

The study notes increasing trends of late-stage cancer among Asian-Americans — particularly among Filipino, Korean and South Asian women, who are the least likely to get screened.

In 2015, Sherry Cava and her three children arrived in the U.S. from her native Philippines to join her mother in Daly City. About a month later, she felt a lump in her breast. Cava, now 44, had never had a mammogram or even routine annual checkups after her three pregnancies, because of the prohibitive cost of health care.

“In the Philippines, when you don’t feel anything, you don’t go to the doctor,’’ Cava said. “It’s only when you’re experiencing constant pain that you go.’’

But after she told a cousin, a nurse at San Francisco General Hospital, about the lump, she was able to get a mammography, ultrasound and biopsy done on the same day, Cava said.

Like Abe-Koga, her breast cancer treatment involved chemotherapy, a mastectomy and radiation. But Cava’s family did have a history of the disease: Her 70-year-old maternal grandmother had been successfully treated for it.

Cava, who completed her last cancer treatments last summer, said she believes if she had not come to America, her cancer would not have been caught as early.

“I am blessed to be here at this moment,” she said.

Reading this on your phone? Stay up to date with our new, free mobile app. Get it from the Apple app store or the Google Play store.