At least one of your slaves is missing the 'is_preg' field in the "preg" dictionary. This is a value is not present in vanilla and should be added by the mod to all slaves. The most likely causes are that you either started the progress or gained a slave in vanilla(compatibility with vanilla is lacking) or added another mod that is causing a conflict. The save file is plain text can be edited using an online JSON editor, though the data problems may extend to many slaves or other areas of the game.
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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.