The news was revealed in a video clip about upcoming episodes of “Keeping Up With the Kardashians,” the E! reality show.
“I just got the blood test back, and I am pregnant,” Kardashian, 34, tells her sister Khloe, who responds with a scream of delight.
The video also shows Kardashian looking at an ultrasound during a doctor’s appointment.
Kardashian and West, 37, already have a daughter, North, who will turn 2 on June 15.
The couple, often referred to as Kimye, got married in Italy last year.
Kardashian has said she that although she didn’t suffer from morning sickness during her pregnancy with North, it was still “really painful everywhere.”
She had made no secret of the couple’s desire to have another child.
“We are trying. We try every single day. You can’t try harder than we try,” she told E! News in late April. “It’s getting exhausting.”
West has repeatedly said that trying for baby No. 2 wasn’t “as fun” as trying for baby No. 1, Kardashian said at the time.
The celebrity family is often in the headlines.
Kardashian’s stepfather, Bruce Jenner, announced in April that he is transitioning to become a woman.
People on Twitter had fun with the news making suggestions for names: