Praise be to Allah.
It should be noted that not every kind of tiredness or hardship is regarded as an excuse which makes it permissible not to fast in Ramadaan, because fasting is not free of some tiredness and hardship, but in most cases it is a small-scale, ordinary kind of hardship.
Based on this, if a pregnant woman feels a little bit tired, one of the following two scenarios must apply:
1 – when she feels a little tired but it does not cause intense, extraordinary hardship, and she does not fear for herself or her unborn child. In this case she is obliged to fast and it is not permissible for her to break the fast.
2 – when the hardship experienced is severe and she fears that she or her unborn child may be harmed. In this case it is better for her not to fast, and that may be obligatory for her.
The fact that a pregnant woman is permitted not to fast is indicated by the words of the Prophet (peace and blessings of Allaah be upon him): “Allaah has waived half of the prayer and fasting from the traveller, and from pregnant and breastfeeding women.” Narrated by al-Nasaa’i, 2274; classed as hasan by al-Albaani in Saheeh Sunan al-Nasaa’i.
If a pregnant woman does not fast, then she is obliged to make up the days when she did not fast. Does she also have to feed poor people for the days when she did not fast because she feared for her unborn child? There is a difference of opinion among the fuqaha’ concerning that.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said, when discussing the differences of scholarly opinion concerning what a pregnant woman must do if she does not fast:
The third view is that she has to make up the fasts only and does not have to feed poor people. This is the most correct view in my opinion, because the situation of pregnant and breastfeeding women is no different to that of sick people and travellers, so they have to make up the fasts only.
From al-Sharh al-Mumti’, 6/362
See also question no. 49794
And Allaah knows best.