It's Not Goodbye by Fatebegins
Agron always returns relatively unharmed from battle. As he prepares to once again fight for the rebels' cause, he promises that this time will be no different. Nasir, on the other hand, has good reason to worry. Not only does he want the love of his life to return safely to his arms, but his soon-to-be-born child needs to grow up with both fathers. When tragedy strikes, Nasir must remain strong for his newborn son as Agron fights for his life.
Short and sweet, this story packs and emotional punch. Your heart will break as you watch Nasir struggle with the fact that Agron may be lost to him forever. That's all I'm going to say concerning the plot, so you should definitely check out this lovely story that portrays sorrow, pain, regret and hope so beautifully. It is m-preg, but it's not squicky, weird or in your face. I know there are some out there that don't particularly care for this trope, but Fatebegins includes it subtly, mainly as a plot device for showing the even greater importance of Agron's safe return from battle.