Someone told me that if a pregnant woman has an uncontrolable craving for a certain food - she is allowed to eat it, even if it is not Kosher. Is this true?

Yes, this is true, but it is talking of an extreme condition where she might be harmed if she doesn't eat. Likewise, a pregnant woman who smelled food on Yom Kippur and her baby might be harmed if she doesn't eat it - we whisper into her ear that it's Yom Kippur today, and if her craving is so great she can't take it, she is allowed to eat. The reason to this is that the situation is one of Pikuach Nefesh (endangering life), and Pikuach Nefesh overpowers the whole Torah. Rabbi Ro'i Margalit