When giving birth.
Viewing post in Aric's Expansion Rebred
You can talk to the headgirl to set it ingame, but that error will be fixed in v0.9.4
Edit: That is an odd error. Somehow it seems that your variable for PregDuration was set to 0. I have it set to check for gestation being greater than 0, though as that is all within a check for the person's pregduration being greater than 0 and that is what sets the gestation that seems odd to have even showed up. Should be fixed now