Sheikh Yusuf Al-Qaradawi states that: It is permissible for an older man/woman not to fast in Ramadan if fasting would be a big struggle for them or it would be an overbearing difficulty. Old people [who cannot fast] have to give a ransom, which is the feeding of a needy person for each day. This is a concession and an easement from Allah. Allah says: (Allah intends for you ease and does not intend for you difficulty.) (Al-Baqarah 2: 185). Ibn `Abbas (may Allah be pleased with him) said: “It is allowed for the old man not to fast. For each day, he is to feed a poor person, and he does not have to make up for the fast days.” According to Al- Bukhari, Allah says regarding the old man and people like him: (And those who are unable have to feed a poor person. But whosoever does good of his own accord, it is better for him.) (Al-Baqarah 2: 184)
So, for the elderly man, elderly woman, and the sick who do not expect recovery from their sickness, they are not to fast and as charity they are to feed a poor person for each day.
as regards a pregnant woman or a suckling mother, if she is worried lest fasting should harm her, the majority of jurists are of the view that she is allowed not to fast, provided that she makes up for the fast-days she missed. A woman in either of those cases is similar to a patient.
Though jurists have unanimously agreed that a pregnant or a suckling woman who apprehends harms to her embryo or her new-born child is allowed to abstain from fasting, they have disagreed as to whether she must make up for the fast days she missed later, or feed one poor person for each day she missed or both. Ibn `Umar and Ibn `Abbas maintain that she is to feed poor people equal in number to the fast days she missed. The majority of scholars are of the view that she must make up for the fast days she missed. Others yet hold that she is to do both. It seems to me that only feeding the poor is enough on its own for a woman who is constantly either pregnant or suckling, so that she has not got an opportunity to make up for the fast days she missed. So it may be the case that a woman is pregnant this year and a suckling mother the next and pregnant again the following year, and so on. She is therefore unable to make up for the fast days on which she refrained from fasting. So if she is commanded to make up for those days, she will have to fast for several years incessantly, which is definitely going to be difficult, and Allah does not want His servants to suffer hardships.