Speaking from personal experience, this is not universally true, but is often the case. I was recently at the wedding of a couple (already had a child) that had converted to Judaism and remarried Jewishly (first Jewish wedding), and they called her "Giyurta Da", and her kesubah sum was 100 zuz. See also DanF's answer above.
However, I heard from a local Rav that in cases where the girl had engaged in relations before she was married, but it was not publicly known, they still write 200 zuz and call her a virgin in the Kesubah, due to Kavod Haberiyos. I would assume if she was visibly pregnant or something along those lines they would not write 200 zuz.
While I don't have an explicit source for this (no access to fancy databases right now, or even a Hebrew Keyboard to google things with), Rav Shternbuch in Teshuvos Vehanhagos 1:755 talks about making Sheva Brachos meals for a couple when she was not a virgin, and allows a "beracha levatalah" so that she will not be embarrassed. See there further for a chiddush of his regarding the definition of a virgin in this regard.