User blog comment:Dragonfly82/The 100 - Female Lead Pregnancies./@comment-24670104-20150916193332/@comment-25176701-20150917201644

"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."

Or maybe there could be somebody who didn't have the implant... Octavia was hidden under the floor for 16 years and eventually was put in prison, where she couldn't get pregnant. Since she was locked up, I doubt the Ark even bothered giving her the contraceptives implant.