Carol's egg cells
It’s really amazing how such a small thing as Carol‘s eggcells (small as in just getting mentioned a few times very casually, not in small bc cells obviously are indeed very small) can serve so many purposes in the story.
First they say so much about Carol and her relationship with Helen and then they obviously become a real driver for the plot when it turns out the hive can use them to get Carol's stem cells.
Carol froze them 2011 so she must in general wanted to have children otherwise she wouldn't have done it. She says "the ones I froze with Helen", so it wasn't just her freezing them for "you never know" but she and Helen wanted children together. So they were happy together and wanted a future with a family... But... They didn't have children... They only froze Carol's eggs.
They had probably already been together for pretty long at that point. We don't know when exactly Carol's books started to be financially successful put if they could afford that procedure at that time they must have been pretty comfortable financially. And even if they put off having children for financial reasons they didn't have them either once Carol did earn quite a lot of money with her books. Writing is also a job that's certainly more compatible with having children than most others : Totally flexible hours, no physical constrains against being pregnant... So maybe Helen was worried about Carol's drinking and wanted her to get a grasp on that first? Or of course there could be other reasons that we just don't know about.
But I think it's more Carol's "You love feeling bad!" and "I'm not good at feeling good...". There is something she wants (otherwise she wouldn't have frozen her eggs) and that she could do (because she has a stable relation ship and she has enough money) but instead of actually doing it she freezes the possibility to maybe some day do it, but never does... For 14 years...
And maybe Carol knew that that that's what she was doing, because maybe Helen installed the motions sensor right after they froze the eggs because Carol started to drink more after that? (Or Helen thought they'd actually use the eggs soon and she wanted to know how problematic Carol's alcohol problem actually was, before they had children. That's also possible of course.)
Maybe she also was scared of that it would bring along more situations where she had to out herself with a child with two moms?
And it's not that wanting children was an idea at some point and then she lost interest in it because in the ice hotel, years after freezing them, she mentions her frozen eggs out of nowhere, so they must be pretty present in her mind all the time. Plus that I think (not sure) that you don't just pay for freezing them but also a continuous fee for having them stored? So if she had lost interest at some point she wouldn't still be paying for them and they wouldn't still be there for the hive to kidnap them...