"Sarah made one slip-up – but [she] dampened suspicion by explaining it away," reports Trailblazer: An Intimate Biography of Sarah Palin, by PEOPLE Assistant Editor Lorenzo Benet and being published Tuesday by Simon & Schuster's Threshold Editions.
Palin also was aware Trig had been diagnosed with Down syndrome but did not disclose this to her four other children until his Trig's 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," according to the book, which was not authorized by Palin.
Trailblazer also reveals 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.
Health Problems at BirthThe biography also reports that Trig's health was problematic from birth – he was born with a congenital heart defect typically found in half the babies with Down syndrome – but it cleared up on its own without surgery.
The Palin home is portrayed as running at a frenetic pace; one-on-one basketball games, take-out pizza dinners, razzing between kids and parents.
Extended family is in abundance, and while they have helped Sarah and Todd Palin raise their children – enabling her to serve as governor and pursue her political career last fall during the presidential campaign – the couple hired a Wasilla nanny to help out at home, the book reveals.
Bristol and LeviTrailblazer also sheds new light on the relationship between Palin's eldest daughter, Bristol, and her fiancé, Levi Johnston, the father of Bristol's son, Tripp, born Dec. 27, 2008.
While Bristol was pregnant last year, she was living in Anchorage with her aunt and uncle, Heather and Kurt Bruce, and working at two espresso shops – while also attending West High School. Levi was 40 miles away in Wasilla, but, "there was certainly no ban on them dating," reports Trailblazer. "Levi used to drive to Anchorage to take Bristol out."
Heather Bruce is quoted as saying in the book, "He used to bring her flowers and take her to dinner and a movie. He would come in and say hello, but he says few words. He's a quiet boy."
Palin, who became an overnight sensation once John McCain tapped her to become his running mate, can fall victim to being star-struck. She once told husband Todd she was going shopping at Costco in Anchorage but detoured to J.C. Penney's to meet Ivana Trump – in town to promote a cosmetics line.
Sarah & Todd Palin balance parenting and politics
Read more about Sarah's family drama