Kanye West and wife Kim Kardashian West. Picture: Bang Showbiz

Kim Kardashian West and Kanye West's surrogate is reportedly happily married with two young sons of her own.

The couple are expected to welcome their third child, another girl, into the world in January after hiring someone to carry the baby for them, and more details have now emerged about the mystery woman.

Sources told TMZ the surrogate and her husband are both African-American and in their 20s, and their five-year marriage is described as "solid".

The woman, a college graduate and registered Democrat, is already the mother of two young boys, and is said to be in "great physical shape".

It was previously revealed Kanye and 36-year-old Kim- who already have daughter North, four, and 21-month-old son Saint - had hired an unnamed woman through an agency to carry their baby for $45 000 (R574 000), which would be paid in 10 equal instalments. It was also agreed the woman would receive an extra $5 000 (R63 000) per child in the event of a multiple pregnancy and an additional $4 000 (R51 000) if she lost reproductive organs in the process.

Read: Kim Kardashian and Kanye West baby 'due in January'

In return, the surrogate agreed that she would not smoke, drink or do drugs as well as refrain from using hot tubs or saunas, handling or changing cat litter, applying hair dye, drinking more than one caffeinated drink per day or eating raw fish.

It was agreed that the woman would restrict sexual activity in the weeks leading up to the pregnancy, including foregoing intercourse for three weeks after embryo implantation.

The agreement also stated the 'Keeping Up With the Kardashians' star - who was advised against getting pregnant again by doctors because of the life-threatening complications she has previously suffered - and her 40-year-old rapper spouse "assume the legal and parental responsibilities for any child ... that may possess any congenital or other abnormalities or defects".