"At one point I thought I had had a miscarriage," she told host Ryan Seacrest during the TV special.
"I was pretty certain and even a doctor told me there was no heartbeat and said 'You have had a miscarriage.'"
"It was around Thanksgiving and it was the worst Thanksgiving ever."
She was shocked when she learned all was OK.
"I was on the monitor and he [the doctor] was like: 'There's a heartbeat!'"
She had a difficult pregnancy with North as she has placenta accreta, and with Saint as well, who was born December 2015.
"Last pregnancy, I had a condition called preeclampsia, which is a serious condition you can get during pregnancy," she wrote in 2015 just before Saint was born.
"It causes your body and face to swell, and that was very uncomfortable for me. I had early-onset preeclampsia and I had to deliver at 34 weeks, almost six weeks early."
For her third child, Kim is reportedly using a surrogate.
Keeping Up with the Kardashians 10 Year Anniversary Special premieres Sunday on E!