"The couple has always wanted a big family, especially Kim," the source says. "She loved growing up with all [her sisters] and Rob. Having big holidays, birthdays and events is what Kim knows and she wanted the same for her own kids."
"Kim and Kanye weighed their options heavily, took time to think everything through," adds the source. "Ultimately they decided to see, if they implanted the embryo, if it would take and left the rest up to fate. Family to Kim and Kanye is everything, and they are ecstatic they are expanding their family."
In addition to Chicago, Kim and Kanye are also parents to daughter North, 5, and son Saint, 3.
Last August, an additional source told ET that although the couple was very happy with their previous surrogacy experience with Chicago, they would be open to trying other methods if it came down to it.
"Kim and Kanye had a positive surrogacy experience with Chicago and have talked about that as the plan for the next child," the source said at the time. "If that unfortunately doesn't pan out, the couple has talked about possibly going through the adoption process. Kim and Kanye know that whatever is meant to be, will be."
But when ET caught up with Kim last month at her new KKW Beauty and Fragrance pop-up shop in Costa Mesa, California, the reality star played coy about her and Kanye's plans for a fourth child.
"Three is a lot, [but] you never know," she said. "My mom [Kris Jenner] kind of prepped me [for what having three kids was like], and it was crazy because when everyone found out that I was having a third, everyone had a crazy story... like, 'Two was great, but three, you are outnumbered. You are exhausted'... Like, all of these nightmare stories," Kim continued. "[Having a brother for Saint] would be nice. It really would, but I'm very full right now."
Hear more from the exclusive chat in the video below.