Sometimes the "catch-up" mode can generate some weird results. It doesn't seem to actually run the game in order, but will run through routines for each type of task. Did you have a male villager set to parenting? When the catch up routine ran, it found an appropriate female to breed with, but when it backtracked the time for how long she had left, it didn't compute that she would have been too young.

One time I noticed that I had six 6-year olds, all with the same mother but different fathers!