No one thought Gwen Stefani and Blake Shelton would last as a couple, but a year-and-a-half into their relationship, the duo is ready to take a major step — and we’re not talking marriage!

“They’re trying to have a baby first,” an insider tells Life & Style. “That’s their main focus right now. [Gwen’s] determined to have another baby and carry it herself, even though Blake has said it doesn’t matter if they adopt or get a surrogate.”

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The source shares that the mother-of-three has been undergoing in vitro fertilization treatments for the past year with no success, and has decided to halt the process, despite her desire to have another kid.

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“Physically and emotionally, it was just a lot for her. So she’s trying to get pregnant naturally for the time being,” the insider reveals.

But the 47-year-old’s first priority remains her sons, Kingston, 10, Zuma, 8, and Apollo, 2, whose father is the blonde beauty’s ex-husband, Gavin Rossdale. “[They’re] still dealing with the emotional fallout of the split,” the insider adds. “Gwen wants to make sure they’re 100 percent ready for her to marry Blake.”

gwen stefani kids with blake splash

Gwen and Blake with her kids.

The “Hollaback Girl” hitmaker certainly isn’t shy about her affection for the country crooner, 40. “It’s so unbelievable. I love Blake. He’s the most incredible guy. Everybody loves Blake and that’s the thing, there’s no way around it,” she said during a recent appearance on Late Night With Seth Meyers. “He’s just a sparkly, happy, amazing person.”

But Gwen also joked that the pair’s competitive natures can cause problems at home, especially while filming their hit TV NBC show The Voice, for which they both serve as coaches. "He’s been on the couch a few nights!” she teased.

For more on your favorite celebrity relationships, pick up the latest issue of Life & Style, on newsstands now. And scroll through the gallery below for a look at the cutest photos from Blake and Gwen’s romance!