A source tells PEOPLE that the couple, who got engaged last year, "decided to try and get pregnant after they had dated for a few months" and started the in vitro fertilization (IVF) process "last summer."
"It's been an emotional rollercoaster. They never expected it to be as hard as it's been," says the insider. "It's been especially difficult for Kourtney. They still would love to have a baby. Travis is not putting any pressure on Kourtney though. He just wants her to be happy and healthy."
Kardashian, 43, is already mom to kids Mason, 11, Penelope, 9, and Reign, 6, with ex-husband Scott Disick, while Barker, 46, is dad to son Landon, 18, and daughter Alabama, 16. He also shares stepdaughter Atiana, 22, with his ex-wife Shanna Moakler.
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However, things were not going as planned for Kardashian, who revealed that she's currently facing complications in her IVF journey, and experiencing depression as a result.
During a visit to Kardashian's home, Jenner sat down with her eldest daughter and asked how her doctor's appointments were going.
"Awful," Kardashian responded, before elaborating in a confessional. "Travis and I want to have a baby and so my doctor took us down this road of doing IVF and it hasn't been the most amazing experience."
Speaking to her mom, Kardashian continued, "Every single person on social media is always like, 'Kourtney's pregnant, Kourtney's pregnant, Kourtney's gained so much weight.' I'm like, it's so rude to comment on people when you have no idea what they're actually going through."
"The medication that they've been giving me, they put me into menopause," she added.
In a confessional, Kardashian added: "I think because I'm so clean and careful with what I put into my body, it's just having the complete opposite reaction and is working as a contraceptive instead of helping us."
The news came as a shock to Jenner, who told her daughter, "I feel like I've never seen you happier so the depression thing surprises me."
In a confessional, Jenner added, "Even though Kourtney has three beautiful children, it doesn't make it hurt any less if you're trying to have a baby and you've been struggling."
Kardashian was in agreement, explaining that the reason she and Barker "want it so badly" is because "we want to make something together."
Kourtney Kardashian Instagram
"Yeah, of course," Jenner responded. "You wanna bring your own baby, your own love into the world. You both know exactly how it feels to have a child and how to bring a child into the world, and now you want to do it with the love of your life."
A trailer for the series also showed the couple at the doctor's office beginning their journey toward expanding their family.
At one point, their doctor asked Kardashian for a "sample" before the reality star quipped, "We'll take our mics off so you don't get the audio." Another shot showed Kardashian lying down during a doctor's visit as the Blink-182 drummer held and kissed her hand.