According to US Weekly, the surrogate is three months pregnant, meaning she's now probably at the stage in which she can go public with the news. This would make the younger sibling to 4-year-old North and 1-year-old Saint due in January 2018.
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Kim Kardashian West's pregnancy struggles have been well-documented in her reality TV show, Keeping Up With The Kardashians. The mother-of-two suffered from a rare condition called placenta accreta during her first two pregnancies, which led to her first born North West arriving premature. During the series, she's been seen investigating other options, after discovering that she's unlikely to be able to bring a third baby to term.
Reports of Kim using a surrogate for her next child have been widely circulated, not least by the star herself, who spoke about it with younger sister Khloe in the most recent season of the show. Tabloids have published various sources quoting crazy sums of money being offered to these potential surrogates, none of which have been confirmed.
Congratulations to the happy couple on their growing family!