Praise be to Allah.
A Muslim woman who misses any fasts in Ramadaan because of being pregnant or breastfeeding must make them up after she no longer has that excuse, just like the sick person of whom Allaah says (interpretation of the meaning): “… but if any of you is ill or on a journey, the same number (should be made up) from other days…” [al-Baqarah 2:184]
A woman may space out these days (i.e., she does not have to fast them all at once, consecutively), as this is easier for her. (See also "Seventy Questions About Fasting "
, under the heading Books on this web-page).
It is better to make up missed days before the next Ramadaan comes, but if the excuse is still present, she may delay making them up until she is able to do so. She should not resort to feeding poor people (instead of fasting) unless she is totally unable to fast. And Allaah knows best.