I'm ashamed that I know this, but Khloe has recently said that this was a lie. She said that she pretended to have fertility problems as, at that time, her partner (Lamar Odom) was having some drug addiction problems and she did not want to have a baby while he was ill. (I don't think she phrased it as nicely as that though).
If I remember rightly, she said that she kept taking the pill to be sure she wouldn't fall pregnant.
I remember this because I have secondary infertility and unsuccessful IVF treatments and was really upset that she would lie about this on TV.
Kim apparently had issues with a retained placenta after North's birth and said that she was told that further pregnancies would be dangerous for her. Three of her children were born via surrogate.