Kanye West and Kim Kardashian have become a family of five after welcoming their third child via a surrogate mother.

Reality TV star Kim announced the happy news in a post on her app on Tuesday (16Jan18).

"Kanye and I are happy to announce the arrival of our healthy, beautiful baby girl," she wrote. "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."

The little girl, whose name has yet to be revealed, was born in the early hours of Monday (15Jan18). She joins Kim and Kanye's four-year-old daughter North, and two-year-old son Saint.

Kim explored the idea of surrogacy after struggling with life-threatening fertility issues during her previous two pregnancies, complications she documented on the family reality show, Keeping Up With the Kardashians.

She confirmed their third child would be carried by a surrogate in September (17), but later admitted she found the process "frustrating".

"It is frustrating like, to not be able to do it on your own," Kim shared in a recent episode of the show, which aired in December (17). "It is a weird dynamic. Like, I go to all the doctor's appointments and I try to be really present, but when you're not carrying it yourself, it is such a fine balance of how much you're going to be in their lives.

"Of course I want to know every last detail that's going on with my baby and that's super frustrating, but I really trust my surrogate and I don't want to be that person that's trying to control her every move. It's going to be a long hard process but I'm going to take it one day at a time and hopefully it just gets easier."

The arrival of the couple's third kid comes as Kim's sister Khloe prepares to welcome her first baby with her basketball star boyfriend, Tristan Thompson. Youngest sister Kylie Jenner is also rumoured to be heavily pregnant with rapper Travis Scott's child.