CARRIE UNDERWOOD is singing a new tune about motherhood after her hockey-playing hubby, Mike Fisher, threatened to call it quits on their marriage!

The two have been going back and forth over starting a family for four years – and the country cutie finally caved. Now, sources say she’s promising there will be a baby on the way by the end of the year.

“She realized Mike was ready to throw in the towel on their relationship over the baby issue,” said a source. “He’s fed up and frustrated and feels betrayed by Carrie.

“It all came to a head earlier this spring when Carrie told Mike that she was mulling several acting offers that would keep her busy and away from home through the end of the year.

“Mike blew a gasket. He told Carrie that she needed to choose between her career and her marriage. Mike has never held her feet to the fire like that before.”

According to the source, Carrie is taking Mike’s threat very seriously.

“She realizes a baby is the only thing that can save their marriage,” the source continued. “After the huge blow-up, she promised Mike that either she’ll be pregnant or they’ll initiate adoption proceedings before 2014 is out.”

The “Jesus Take the Wheel” beauty has been working nonstop since she shot to fame after winning Season 4 of “American Idol” in 2005, scoring 18 No. 1 hits and topping 45 million in record sales.

But Carrie, 31, and Mike, 33, who married in July 2010, “have been at each other’s throats for more than a year” over when to have kids as well as outrageous rumors that Carrie was having an affair with fellow country singer Brad Paisley.

In fact, as The ENQUIRER reported, friends feared she was headed for a breakdown because of the stress.

But “Carrie seems to have finally come to her senses,” said the source. “She’s ready to settle down and make family her priority.”

Although the couple will be trying to conceive naturally, they’re also open to adopting.

“Mike doesn’t care how a baby comes to them,” the source added. “All he cares about is that they become parents soon.”