For the past few years, I've been hearing that more women are giving birth via c-section, and that this is in large part due to biological necessity from their small hips. This could become a problem, if women who birth daughters via c-section wind up creating a line of women who in turn can only deliver via c-section. FWIW, most of the women I know who gave birth via c-section wound up not having any more kids after that. I've also heard that women who are emotionally/physically stressed and forced to have lower body fat percentages see their fertility drop off a cliff. So I think you're right about these models not looking like they could withstand a pregnancy, assuming they could even get one.