It was clear that Moriarty intended to destroy Sherlock; it had been explicitly stated by the Alpha, and it was becoming obvious that the only way to save himself - save everyone - was to remove that incentive.
Soon, Moriarty would be at 221B Baker Street; if Sherlock timed it right, he would soon go into oestrus, and no Alpha could resist a virgin Omega who, until now, had successfully masqueraded as a Beta, and would be ultra-fertile.
Removing the now-empty syringe from his arm, Sherlock could feel the hormones already flooding his system and bringing on his long-delayed heat, and began to prepare himself mentally to carry Jim Moriarty's progeny.
My second three-sentence fic. The idea of Sherlock purposely getting pregnant to stop Moriarty killing him appealed to the darker side of my personality. I could definitely consider writing a sequel to this, if people were interested.