The show gives absolutely no information about Robin's medical disorder. All we know is that she thought she was pregnant by Barney, had a test, and was informed by her doctor that she will NEVER be able to have children.
This seems like such a rare disorder, because with most issues that lower fertility, there is still a slim window of opportunity. I'm thinking that for a doctor to be able to tell her, so definitively, that it can never happen, she must have been born without any eggs.
Even if she were going through early menopause, doctors always tell women in menopause to consider that they could become pregnant up until the point at which they haven't had a period for a year. And Robin's period was late but not that late.
Can anyone else think of any medical condition, besides not having eggs or being completely through menopause, that can render a woman totally incapable of having a baby? I've even heard of an occasional pregnancy surviving outside the uterus, so it seems that not even that is 100% necessary.