The type of cancer has not been disclosed. She started chemotherapy in late February.
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READ MORE: Kate Middleton says she was diagnosed with cancer, is undergoing chemotherapy
Kate Middleton diagnosed with cancer, undergoing chemotherapy
The announcement sparked an outpouring of support and comes as her father-in-law, King Charles, King Charles, also undergoes cancer treatment.
"This of course came as a huge shock, and William and I have been doing everything we can to process and manage this privately for the sake of our young family," Kate, 42, said in the video, which was filmed at Windsor on Wednesday. "As you can imagine, this has taken time. It has taken me time to recover from major surgery in order to start my treatment."
The cancer was discovered in post-operative tests after her abdominal surgery in mid-January, Kate said.
Kate described it as a "major abdominal surgery," and said in her video, "at the time, it was thought that my condition was non-cancerous."
TIMELINE: A look at Kate Middleton's cancer diagnosis, surgery, absence from public duties
The type of surgery she underwent has not been released.
"The surgery was successful," Kate said. "However, tests after the operation found cancer had been present. My medical team therefore advised that I should undergo a course of preventative chemotherapy and I am now in the early stages of that treatment."
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ABC News report on Kate Middleton cancer diagnosis announcement
At the time of Kate's surgery, Kensington Palace said her condition was non-cancerous. The palace has provided little information on her condition since.
It's a shocking discovery for the Royal family and people in Pennsylvania.
"It's so sad to hear someone go through that," said Anthony Kuhn from Reading. "It's such a devastating disease."
"It is very shocking," said Taylor Priore from Royersford. "She is very young. It is very upsetting."
Dr. Andrea Porpiglia, who is an associate professor at Fox Chase Cancer Center, weighed in on the diagnosis.
"It sounds like she had pretty major abdominal surgery, and the healing process can be 6-8 weeks for that -- that's not unexpected. She's doing exactly what I would tell my patients - increasing your daily activity, making sure you're nourishing yourself, and taking care of your mindset."
Dr. Porpiglia believes the treatment is a standard of care.
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"She was diagnosed with cancer and it hasn't spread, it hasn't metastasized," said Dr. Porpiglia. "But in order to prevent that from happening and prevent a recurrence, they're recommending chemotherapy for her."
Dr. Porpiglia said cancer among adults under the age of 50 is on the rise.
"Unfortunately, over the past decade, we've seen it with breast cancer and colon cancer," said Dr. Porpiglia. "We actually changed our screening guidelines for colon cancer because people are being diagnosed at an earlier age."
Kate will return to official duties when she's cleared by her medical team.
King Charles is "so proud of Catherine for her courage in speaking as she did," a Buckingham Palace spokesman said.
The king and the Princess of Wales were both in the hospital at the same time in January. And in the last few weeks, the king has "remained in the closest contact with his beloved daughter-in-law," the spokesman said.
King Charles and his wife, Queen Camilla, "will continue to offer their love and support to the whole family through this difficult time," the spokesman said.
Prince Harry and Meghan, the Duke and Duchess of Sussex, said in a statement, "We wish health and healing for Kate and the family, and hope they are able to do so privately and in peace."
The Sussexes' comments come amid a yearslong rift between Harry and William.
ABC News contributed to this report.