Abby had waited the last possible moment to tell him. While she was glad most days to be alive, she had not forgiven him for saving her; and has she learned later saving them. On the Ark the mere thought, even if a fantasy of having a second child was absolutely ridiculous. They simply weren't wired to consider that, with a few exceptions over the years nobody did.
When it became clear she was carrying another child, it almost felt like the universe was giving her not only a replacement for Clarke, but Jake as well. Then it became emotionally too tiring. Marcus, she cared for him, even loved him, but her life with Jake counted for something. This child growing inside represented the official new start, Abby wasn't ready.
It hadn't been hard too conceal the growing bump and Marcus respected her wishes of staying the hell away from her. With him fighting an endless battle with Octavia and the decisions she constantly tried to pass upon the bunker, Abby knew he was always preoccupied. She had been surprised that somehow she made it seventeen weeks before she knew the secrecy and the prayers that passerby's won't reveal the news to him was gonna crumble.
His reaction had been calm, almost numb, she imagined hidden resistantment towards her for not allowing him to be apart of this journey, but he said not a single negative word. If Abby had been wanting more out of Marcus she wasn't sure.
This was life now. Her. Marcus. A baby that a year ago won't have been possible. Total miracles and blessings, but life still felt like hell. Only if he had respected her wishes and had allowed her to die. Somewhere out in the void of the universe waited Jake and Clarke.