Praise be to Allah.
If the woman who is pregnant -- and likewise the woman who is breastfeeding -- fears for herself or her child, she may break the fast in Ramadan, and she only has to make up those days, because she comes under the same heading as a sick person who is excused because of his sickness. This has been discussed the answers to questions no. 50005 and 49848.
Then if she is able to make them up before the next Ramadan, she is obliged do that and it is not permissible for her to delay until the next Ramadan begins. If her excuse remains in effect because of a new pregnancy or breastfeeding or sickness or travel, until the next Ramadan begins, then there is no sin on her and she has to make it up when she is able to do that.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have mercy on him) was asked:
There was a woman who did not fast in Ramadan because of nifaas (postpartum bleeding) and she could not make up the fasts because of breastfeeding, before the next Ramadan came. What does she have to do?
What this woman has to do is fast to make up the days when she did not fast, even if that is after the following Ramadan, because she only refrained from making them up between the first Ramadan and the next because of this excuse. But if it is not difficult for her to make them up during the winter, even if it is day after day, then she has to do that, even if she is breastfeeding. So she should be keen to do as much as she can in order to make up last Ramadan before the next Ramadan comes. But if she cannot do that, there is nothing wrong with delaying it until the next Ramadan comes. End quote.
(19/answer to question no. 360)
He (may Allah have mercy on him) was also asked:
There is a woman who did not fast the month of Ramadan because of childbirth, and she has not made up that month. That was a long time ago and she is not able to fast. What is the ruling?
What this woman has to do is repent to Allah from what she has done, because it is not permissible for a person to delay making up Ramadan until the next Ramadan comes except for a legitimate shar‘i excuse. So she has to repent, then if she is able to fast, even if it is day after day, then she should fast. If she cannot do that, then she should see: if it is for an ongoing reason, then she should feed one poor person for each day. If it is for a temporary reason that it is hoped will disappear, she should wait until it disappears, then she should make up the days she owes. End quote.
(10/answer to question no. 361)
The questioner did not state the reason for her sister being unable to make up the fasts. If the excuse is temporary and it is hoped that it will come to an end (pregnancy, breastfeeding or sickness), then she has to make it up if she is able to do so.
If the excuse is permanent because of chronic sickness for which there is no hope of a cure, then she does not have to make up, but she has to feed one poor person for each day.
And Allah knows best.