Fasting on these days is only mustahabb (recommendable), not obligatory to be made up for when missed. By missing the prescribed meritorious days one cannot get them back as the reward is restricted to fasting on these specific days.

However, if a person has an excuse for not fasting on all or some of these days, though he had the habit of fasting on, as in cases of sickness, travelling, having menses, then he/she will hopefully get the reward for his/her intention as mentioned in a hadith that reads “When a servant of Allah falls sick or sets on a journey, he will get reward similar to that he gets for good deeds he used to do when he was healthy or at home.” A woman in her menses falls under the same category as the impediment in all such cases is something beyond one’s capacity.

Allah Almighty knows best.