Hmmm I read something about embryonic diapause in rats, where the eggs can be fertilized but not actually implant and then implant later. Its not crazily common but does happen apparently. Maybe that's what's been going on?

I also read that rats can retain sperm, supposedly for a long time and that female rats can occasionally have 2 uteri.
I'm not 100% which female had either of the second sets but 2 of the same females were parenting together both times so it could very well have been the same female both times.
Both times the second set of babies was born 2 weeks after the first.

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