A new biography of Alaska Governor Sarah Palin claims she initially kept her last pregnancy secret -- even from family members.

The book -- by People magazine's Assistant Editor Lorenzo Benet and being published Tuesday by Simon & Schuster's Threshold Editions -- says her undisclosed pregnancy with son Trig was almost discovered when one of her daughters found the child's prenatal ultrasound scan and confronted her mother with the picture.

"Sarah made one slip-up - but [she] dampened suspicion by explaining it away," the book, Trailblazer: An Intimate Biography of Sarah Palin, claims, according to People magazine.

The book, which was not authorized by Palin, says the former vice presidential candidate also was aware that Trig had been diagnosed with Down syndrome but did not disclose this to her four other children until his birth last April 18, several weeks after she went public with her pregnancy.

"Not discussing the pregnancy with her daughters, she felt, would shorten the process and spare them from unwanted attention," the book says.

Trailblazer also purports another reason Palin kept the pregnancy quiet. She tells Benet, who interviewed Palin and her family for a 2008 People cover story: "Not knowing in my own heart if I was going to be ready to embrace a child with special needs - I couldn't talk about it."

In the course of preparing the book, Benet received the cooperation of Palin's parents, Chuck and Sally Heath; the governor's older sister, Heather Bruce; as well as in-laws, political allies and rivals.