Sterilization, or a vasectomy, is required for all male astronauts on the Antares mission. Donner undergoes this after his return from the Mars Mission, though Claire's physical (and Zoe's pregnancy) proves that it was not successful. Claire claims the process is quick and painless, but Donner says that is not the case. ("Pilot")

However, this seems to be contradicted by Rollie's comments that upon Jen's return there would be plenty of time for the two of them to have babies. ("Natural Selection")