Sa'id bin Jubayr (may Allah have mercy on him) reports that Ibn Abbas (may Allah be pleased with them both) said: A concession was given in this to the elderly man and the elderly woman, even though both of them were able to fast: they were permitted to break the fast and, if they wished, to feed a poor person in place of each day. Then this permission was abrogated by this verse: "So whoever among you witnesses the month, let him fast it" [Surah al-Baqarah: 185]. Then the permission remained for the elderly man and woman when they are unable to fast; and the pregnant woman and the nursing woman, when they feel fear, may break the fast and feed a poor person in place of each day. In one narration the word "hubla" (pregnant) is used.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8077