It's not the fact that Nerys had carried Keiko's baby when she hadn't been able to. It's the fact that the experience showed her a side of the irascible major that she wouldn't otherwise have seen.
It was a side she liked.
A side she wanted to see more of.
So she began to invite the other woman to dinner, or lunch, or anything else she could think of. Anything to keep their relationship from slipping away after Kirayoshi's birth. It was hard, juggling two small children and a husband, and this new thing with Nerys.
But the first time they kissed - a ghosting little thing, with the barest hint of pressure and followed by shy smiles - she knew it had been worth every moment.