Olivia Munn revealed her previously private breast cancer diagnosis Wednesday − and her story serves as a crucial reminder about the prevalence of the disease and the importance of early detection.

Munn, 43, shared on Instagram she's had four surgeries over the past 10 months and has spent so many days in bed she's lost count. Thirty days after a biopsy, she had a double mastectomy.

"Surprisingly, I've only cried twice," the actress wrote. "I've kept the diagnosis and the worry and the recovery and the pain medicine and the paper gowns private. I needed to catch my breath and get through some of the hardest parts before sharing.

Breast cancer is a pernicious disease that can be tricky to catch early. Here's what to know about detection.

How did Olivia Munn find out she had breast cancer? What is a mammogram?

Munn described taking a genetic test in February 2023 in order to be "proactive about my health," during which she tested negative for 90 different cancer genes. She also said she had a normal mammogram that winter but was diagnosed with breast cancer two months later.

A mammogram is an X-ray image of a person’s breasts. Mammograms are typically used to look for signs of breast cancer, according to the U.S. Centers for Disease Control and Prevention. They can also be used to examine changes in a person’s breast, such as a new lump, pain in the breast, nipple thickening or discharge and more.

Munn said she was sent to get an MRI after a doctor found her lifetime risk for breast cancer was 37%, and a biopsy found she had Luminal B cancer, an "aggressive, fast moving cancer," in both breasts.

"I'm lucky," Munn wrote. "We caught it with enough time that I had options. I want the same for any woman who might have to face this one day."

More:Olivia Munn reveals breast cancer diagnosis, underwent double mastectomy

What is the Breast Cancer Risk Assessment Tool?

Though Munn said she wasn't scheduled for her routine mammogram for another year, her OBGYN decided to calculate her "Breast Cancer Risk Assessment Score," looking at factors like Munn's age, her familial breast cancer history and the age she had her first child.

"Because of that score I was sent to get an MRI, which led to an ultrasound, which then led to a biopsy," Munn wrote.

According to Mayo Clinic, the Breast Cancer Risk Assessment Tool is a statistical model that predicts a woman's risk of developing breast cancer in the future, following a breast biopsy showing benign findings. The tool is meant to be used by healthcare providers to help determine a patient's options for breast cancer surveillance and prevention. It's free to take on Mayo Clinic's website, but it should not be used alone to determine medical treatment and is not a substitute for treatment from a healthcare provider, Mayo Clinic says.

"A number of validated breast cancer risk assessment tools are readily available online and can be completed quickly in an office setting," reads a practice bulletin from the American College of Obstetricians and Gynecologists on "Breast Cancer Risk Assessment and Screening in Average-Risk Women." "Some tools are better for certain risk factors and populations than others."

More:Katie Couric talks colon cancer awareness, breast cancer diagnosis and becoming a grandmother

Breast cancer cases in women under 50 on the rise, experts worry

While breast cancer in young women remains quite rare, experts worry the number of breast cancer cases among women younger than 50 is on the rise. Ending a longstanding debate over when women should start having mammograms, the U.S Preventive Services Task Force in May of last year joined many other organizations in recommending that women of average risk begin screening at 40. Previously, the group advised women to start at 50, but evidence that breast cancer is increasing in women in their 40s helped prompt the change.

Estimates suggest that between 4% to 7% of women in the U.S. who are diagnosed with breast cancer are under the age of 40, however, so doctors urge that women do regular breast exams to detect any changes. Women at age 30 have a 0.5% chance, or 1 in 204, of being diagnosed with breast cancer compared to about a 1.6% or 1 in 65 chance one decade later, according to the National Cancer Institute.

In her role as medical director of the breast care program at Community Health Network in Indianapolis, Dr. Erin Zusan previously told USA TODAY she's seen more young women being diagnosed with the disease. Lifestyle changes such as increased alcohol consumption and obesity contribute, along with better imaging to detect cases earlier, Zusan said.

The American Cancer Society recommends that women ages 40 to 44 should have the choice to begin annual breast cancer screenings with mammograms, and women ages 45 to 54 should get mammograms every year. 

Some women with genetic mutations may start screenings as early as age 25. Women with a family history of breast cancer can also screen starting "10 years earlier than the first affected relative in the family," according to the Cleveland Clinic.

More:Katie Couric was diagnosed with breast cancer after a mammogram. Don't skip your routine test

How important is early breast cancer detection?

Early detection can be critical, and doctors urge women to do one self-breast exam one day each month.

If women detect any change, they should watch it for a week or two or through one menstrual cycle and if something continues to feel off, contact a health provider, Zusan said. Young women may shy away from reaching out, afraid that they’re overreacting, she added. That should not detract them.

"You’re not wasting our time. We would rather give you good news and explain what they’re feeling if it’s not cancer,” she said. “We don’t want to ignore something that someone feels is a change from their normal.”

More:At 25 she found out she had the breast cancer gene. Now, she's grieving motherhood.

Contributing: Brendan Morrow and Marina Pitofsky, USA TODAY; Indianapolis Star, USA TODAY Network