Is it obligatory for one who missed two consecutive Ramadans due to a legitimate Shar'i excuse (breastfeeding and pregnancy with hardship) to make up for two months of Ramadan with expiation, or is expiation only required?

There is no expiation (kaffarah) for a woman who delays making up fasts if the delay is due to a legitimate excuse, such as illness, pregnancy, or breastfeeding. She is only required to make up the fasts after the excuse is removed. There is no fidyah (ransom) upon her if her excuse continues until the beginning of the next Ramadan. There is also no feeding (it'am) required of her if she broke her fast out of fear for herself; she only needs to make up the fasts. However, if she feared for her child, then according to the majority of scholars, she is required to both make up the fasts and feed the poor.