User blog comment:Dragonfly82/The 100 - Female Lead Pregnancies./@comment-24670104-20150916193332/@comment-26247132-20150917161822

Besides, the writers already said everyone is implanted with contraceptives on the Ark. Only way for one of them to get pregnant is if that fails, in which case, it's just adding pregnancy to create drama and not because it's actually necessary to the story.