Have a technical question you can't find the answer to in FAQ's? Ask it here. Game problem questions that do NOT pertain to things like training advice, what would my horse do well in, how much is it worth.
Use the "Take Turn (Next Day)" button up at the top. Mares will only foal if they are in the stable. They will not foal (or age) in the pasture. Generally foals arrive two turns after the "due date".