According to the Maternity Benefit Act, "The maximum period for which any woman shall be entitled to maternity benefit shall be twelve weeks, that is to say, six weeks up to and including the day of her delivery and six weeks immediately following that day." This leave, which she has requested, has nothing to do with the act. You can allow her to go on leave based on her condition, which might be leave with pay or without pay, based on your management discretion.
During pregnancy, there might be a lot of complications, so please act judiciously. Also, speak with her and understand her conditions; some employees may even take a break from work after delivery. You should be worried if she extends her leave.