"North and Saint are especially thrilled to welcome their baby sister," says Kim.
"Kanye and I are happy to announce the arrival of our healthy, beautiful baby girl," Kim announced on her official website. "We are incredibly grateful to our surrogate who made our dreams come true with the greatest gift one could give and to our wonderful doctors and nurses for their special care. North and Saint are especially thrilled to welcome their baby sister."
According to Kardashian, the baby was born at 12:47 a.m. and weighed in at 7 lbs. 6 oz.
Kim previously made it clear on the family's reality TV series, "Keeping Up With the Kardashians," that she wanted to expand her family but wasn't able to do so since she suffered from placenta accreta during her last pregnancy.
"My doctor had to stick his entire arm in me and detach the placenta with his hand, scraping it away from my uterus with his fingernails. How disgusting and painful," she wrote on her blog. "My mom was crying she had never seen anything like this before. My delivery was fairly easy, but then going through that -- it was the most painful experience of my life. They gave me a second epidural, but we were racing against time, so I just had to deal."
"Knowing that I was able to carry my first two babies and not, you know, my baby now, it's hard for me," she also explained to ET. "It's definitely a harder experience than I anticipated just in the control area."
So far, Khloe Kardashian is the only one in the family to celebrate the baby's birth on social media, writing:
💕💕💕💕💕💕 https://t.co/ah1b4wwvRm— Khloé (@khloekardashian) January 16, 2018