Fantasy Contraception: Full fertility control is built right into the genome of each alpha baseline, and therefore into every clade built on top of that alpha baseline. The male reproductive system does not produce gametes and the female one does not cycle (or their alternate-species equivalents) until instructed otherwise by the proper pharmaceutical and/or biofeedback impulses. And, importantly, default is off. No accidental children, m’kay?
(Of course, most of the time it’s just off and stays off, since actually producing children in vivo rather than in vitro is the sort of appalling, painful, inconvenient, medically hazardous (for both parent and child), etc., etc., relic of a barbarous past that civilized societies don’t expect people to do. The Imperials go even further than the Vorkosigan universe’s Betans in pointing this one out.)