Is there any other way to compensate for missed fasts due to pregnancy and breastfeeding apart from fasting those days at a later time?

If a woman misses several years of fasting the month of Ramdan due to pregnancy and breastfeeding is she obligated to make these up by fasting each day? Is there another option that she may take such as feeding the poor because it would be very difficult for her to make up so many days? Some women are pregnant or breastfeeding for many years in a row without an opportunity to make them up. Also, does she need to make these up before she can do voluntary fasting such as during the month of Shawwal? If this is the case, then it may be difficult for her to gain the rewards of fasting during this month since she would need to make up the missed days first ? Is the opportunity to make up days lost after a certain time period ? For example, if the woman had not made them up before the next Ramadan is the opportunity lost ?