If a villager is in a very weakened state (i.e. less than 10) they shouldn't initiate or accept a mating (don't make them run away... just have a "they are too weak for this" status like if they were sick.
I opened a game this morning that was on 2x speed. Many of my elderly villagers had died, including a doctor. Therefore I had lots of sick and weakened villagers.
I quickly started training new doctors and healed a few of the sick villagers, which also raised their "weakened" status. However, the GC called a party and the rest were "cured" but many were still very weak. (GC curing doesn't seem to change the level just change "sick" to "healthy", "elderly" or "weakened.")
But before I had a chance to stop it, a Master Stud Daddy found one of the elderly grandmoms and City started nursing a new baby at 71! The problem is that she also had a status bar of "empty" (was probably only a 1 or 2.) Since she won't go to the food bin to eat, it looks like she'll be very weak for the entire nursing period and might not make it to the end.
I needed to leave for work, but I didn't leave the game on pause, so I probably won't know how this will turn out. I was just feeling really bad for poor City.
I don't mind so much the child at the late stage, but if she is pending two years caring for the baby, she should at least also be able to care for herself or not have the baby if she it too weak to survive the entire two years.