On the Ark, having a second child is illegal. Which can result in the parents of the second child getting floated, and the second child being locked up until they were old enough to get floated too, leaving the first born without a family. At least, unless you were one of the privilaged.
Abigail Griffin knew the rules, so she wasn't entirely pleased when she found out she was pregnant with a second child. Her first born, Clarke, wasn't even a year old. When she and her husband confessed to the Chancellor, a close friend at the time, he gave them a choice. Basically, it was either kill the unborn child, or let her live her life with her family until she turned twelve. Once she turned twelve, into the skybox it is.
As much as I wish I did, I do not own any of The 100. All rights go to Jason Rothenberg. I don't own any of the characters except for Annalia Griffin and maybe a select few I make up along the way. This story won't follow the exact story line, as I am adding in stuff.