"While they're excited for a family of four, they're cherishing their last Christmas as a family of three," a source tells PEOPLE of Carrie Underwood
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Carrie Underwood
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Carrie Underwood is getting ready for her final Christmas before baby.

The country singer, 35, is currently pregnant with her second child, another boy, and will spend the holiday making memories with husband Mike Fisher and their firstborn, 3½-year-old son Isaiah Michael.

“While they’re excited for a family of four, they’re cherishing their last Christmas as a family of three. Unless of course the baby makes an early debut,” a source tells PEOPLE.

The main hint Underwood has given about baby’s arrival date is that her Cry Pretty tour will kick off in May 2019, after she has finished her maternity leave.

“Carrie is on lockdown until the baby [is born],” the insider tells PEOPLE. “They’re fully moved into the Leiper’s Fork house and have the hospital route mapped out.” (Leiper’s Fork is a rural suburb of Nashville, Tennessee.)

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Carrie Underwood
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The 52nd Annual CMA Awards - Arrivals
Carrie Underwood and Mike Fisher
| Credit: John Shearer/WireImage

At the end of November, the American Idol champ joked that she’s at the stage in her pregnancy where her entire wardrobe comes from the closet of husband Fisher, a former ice hockey pro.

“Officially wearing Mike’s clothes in public now, so … 😳 #pregnant #MyClothesDontFit #sorrybabe,” Underwood wrote on Twitter. Shortly after, Fisher quipped back, “How dare you stretch my clothes 😂.”

Prior to revealing her baby’s sex for the first time publicly while hosting the 2018 CMA Awards on Nov. 14, Underwood surprised fans in August when she announced that she and Fisher, 38, are expecting their second child.

“You might be wondering or asking, ‘Carrie, why is your tour starting in May?’ Well … yay!” she said in an Instagram video before revealing balloons spelling out “BABY” above her head. “Mike and Isaiah and I are absolutely over the moon and excited to be adding another little fish to our pond.”

2018 CMT Artists of the Year
Carrie Underwood
| Credit: Terry Wyatt/WireImage

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The happy baby news was the light at the end of the tunnel for Underwood, who revealed in September that she suffered three miscarriages over the last two years.

The “Before He Cheats” singer previously told PEOPLE that her little boy’s world “is about to get rocked” with the addition of a baby brother.

“[Isaiah] told me recently that when the new baby comes that Daddy could have that baby and then he could be mine,” she said. “Isaiah is my guy.”

Carrie Underwood
| Credit: John Shearer/Getty

“He loves me,” she continued. “He looks at me and talks to me and interacts with me like nobody else on this planet. He’ll stare at me and I’m like, ‘What are you looking at?’ And he’s like, ‘I just love you.’ What kid does that?”

Underwood’s dynamic with Isaiah had her wondering if their second child will be a Daddy’s boy.

“Maybe this next one will be all Mike’s,” she said. “Maybe that will be how it works out. Mike needs some love too. I feel bad for him sometimes.”