First Born is a short story by the pseudonymous jespah.
The story is told from the perspective of Richard Daniels, as he grapples with the consequences of unprotected sexual relations with Hoshi Sato (mirror).
In January of 2156, Daniels carries out a mission wherein he visits the USS Defiant (NCC-1764) and performs some repairs. As a payment for his successful work, Hoshi grants him bed privileges. He departs, thinking nothing has changed.
However, his sister, Eleanor Daniels, a docent at the Temporal Museum on Lafa II, gives a lecture on the royal children of the Empress. In the middle of her talk, she unknowingly changed her story, from five children, to six.
The Empress had become pregnant, and had a son, Jun Sato. Jun became the first born and the heir apparent, instead of Kira Sato, who would be the son of Hoshi and Aidan MacKenzie (mirror).
In Daniels's time, the Mirror universe's government is furious. They send a representative, Ray Jimenez, to demand that Jun's conception be undone. Rick and his superior officer, Admiral Carmen Calavicci, negotiate with Jimenez in order to suffer Jun to live.
Conditions allowing Jun's survival include getting Crewman Elizabeth Cutler (mirror) off the Defiant, sterilizing Jun, and forbidding Rick from visiting the mirror universe at any time during either Hoshi or Jun's lives, ever again.
Caught between a rock and a hard place, Rick agrees to these terms, and could never, ever see his own temporally paradoxical child.