Slaves impregnated in vanilla gameplay will have a baby ID in the "baby" field but will have "is_preg" still set to false. Slaves impregnated in this mod will have at least one baby ID in the "unborn_baby" field and the "is_preg" entry set to true. However, if the slave was added during vanilla gameplay but not expanded to have the addition data for the mod, then the "is_preg" entry will be missing, which is your primary problem.