Honestly I don't know the exact situation, but if they are anything like my ex who was all of the lazy vampire meme, their real fear wasn't motherhood, but responsibility. Her opinions made the choice between having a family or a career focused life very easy for me, and it was a bitter breakup. However, she did end up deciding she wanted kids a few years later, but spent the time inbetween our break up doing the typical "find yourself" routine. Got back in touch with me afterwards saying I had never given her the chance to try having kids, that she "wasn't ready" at the time, and that I had been impatient. Seriously, if you're financially supporting your SO and they aren't working to get ahead in their life, there should have always been the understanding it isn't simply a free ride. Back to your dilemna, I wouldn't phrase it as "I want kids now, give uterus.", but rather asking her how she fits into your plans for life, and if she doesn't want to advance her career, what she's going to be doing instead. Make it her idea, even if you aren't giving her many options. Also, don't think because of her opinions on having children now, that things can't change. I've seen more than a few women be very happy about it after the fact rather than immediately before.
Yeah, fickle thing that women telling you "I want to have your children." doesn't exactly mean "I want to be a mother." Trying for a surprise pregnancy is a really shitty idea.