I don't remember a lot of details but I remember the human race ends up allying itself with a seemingly friendly alien race that agrees to give humanity advanced medicine and technology, but with the goal of slowly wiping out humanity through slowly making the species sterile. It follows a couple that is trying to conceive but has so far been unable to and the overlords keep reassuring them thR everything is fine just keep trying but the woman does digging and figures out what is happening but it's already too late. I think there is a time travel story where humanity is prevented from either coming into contact with the aliens in the first place or is warned away from making the deal.

Thank you thank you, it's driving me crazy.