Is this before you finish the sibiling quest where you go to the hideout to save your brother? Also is your game modded at all and was the date of birth for that child about 36 days for base pregnancy days?
While I was testing I left the game running for a while after I used a command to conceive a child and wasn't able to catch the time it took for her to give birth as the time stamps for when she got pregnant are gone. The last recorded time she got pregnant I saw was Autumn 12, 1085 and this is a vanilla game.