Kim Kardashian West and Kanye West have named their new-born daughter, delivered by surrogate on Monday, Chicago West.
Kardashian West said that not carrying the child herself was "so hard", but that the couple had an instant connection with her.
West was raised in the US city Chicago.
Kardashian West carried and gave birth to her older children but was advised against getting pregnant again because of possible medical complications.
Her daughter North is now four years old and her son Saint is two.
But she suffered from pre-eclampsia and placenta accreta on those occasions.
Pre-eclampsia is a condition that usually occurs in the second half of pregnancy and can cause serious complications including seizures and death if not monitored and treated.
Placenta accreta is when the placenta attaches itself too deeply into the wall of the uterus and remains attached after childbirth.
Kardashian West, a reality TV star, said in a post on her app that "doctors said that it wasn't safe for my, or the baby's, health to carry my own".
The 37-year-old said she and her rapper husband decided to use a "gestational carrier", meaning the baby is "biologically mine and Kanye's".
She wrote: "Having a gestational carrier is definitely different, but anyone who says or thinks it's the easy way out is completely wrong.
"People assume it's better because you don't have to deal with the physical changes, pain or complications with delivery, but for me it was so hard to not carry my own child, especially after I carried North and Saint."
Kardashian West's surrogate was discussed on her reality show Keeping up with the Kardashians, but her identity was not revealed.
A 'special' experience
The star said she had a "great" relationship with the surrogate, and "trusted her completely throughout the entire process".
She added: "I'm so grateful for modern technology and that this is even possible. It's not for everyone, but I absolutely love my gestational carrier and this was the best experience I've ever had.
"Having a gestational carrier was so special for us and she made our dreams of expanding our family come true."
The Keeping Up With The Kardashians star announced that her daughter was born at 12.47am on Monday and weighed 7lb 6oz.
Meanwhile, a lawsuit filed by Blac Chyna against Kim Kardashian West and mother Kris Jenner has been "tentatively" dropped, according to TMZ.
Blac Chyna, who has a daughter with Kim's brother Rob, was suing the pair after claiming they stopped the second series of E! show Rob & Chyna from being commissioned.
The show documented the birth of their now one-year-old daughter Dream and was not renewed for another series.
In December, lawyers for Jenner and the Kardashians filed for a dismissal of the lawsuit against them, which a judge has granted because there was no contract for the show's renewal.