A pregnant woman is obliged to fast, just like anyone else, unless she fears for herself or for her baby, in which case it is permissible for her not to fast.
Ibn ‘Abbaas (may Allaah be pleased with him) said, concerning the verse (interpretation of the meaning):
“And as for those who can fast with difficulty, (e.g. an old man), they have (a choice either to fast or) to feed a Miskeen (poor person) (for every day)”
“This was a concession granted to old men and old women who are able to fast but it is difficult, so they are allowed not to fast, and must feed one poor person for each day; and for those who are sick or pregnant, if they fear for their children, so they are allowed not to fast and must feed (one poor person for each day).” Narrated by Abu Dawood, 2317; classed as saheeh by al-Albaani in Irwa’ al-Ghaleel, 4/25, 18.
It should be noted that it may be permissible, obligatory or haraam for a pregnant woman not to fast.
It is permissible for her not to fast if fasting is too difficult for her, but will not harm her.
It is obligatory for her if fasting will cause harm to her or her baby.
It is haraam for her if no hardship results from her fasting.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said:
In the case of a pregnant woman, one of two scenarios must apply:
1 – She is strong and active, and fasting does not cause any hardship to her or affect her baby. This woman is obliged to fast, because she has no excuse for not fasting.
2 – When a woman is not able to fast, because the pregnancy is too difficult for her, or because she is physically weak, or for some other reason. In this case she should break the fast, especially if it will cause harm to her baby – in which case she is obliged not to fast.
Fataawa al-Shaykh Ibn ‘Uthaymeen, 1/487
Shaykh Ibn Baaz (may Allaah have mercy on him) said:
Pregnant and breastfeeding women come under the same ruling as those who are sick. If fasting is too difficult for them, it is prescribed for them to break the fast, and they have to make up missed fasts when they are able to do so, just as in the case of one who is sick. Some of the scholars were of the view that it is sufficient for them to feed one poor person for each day missed, but this is a weak (da’eef) view. The correct view is that they have to make up the missed days, as is the case of those who were sick and for travellers, because 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”
This is also indicated by the hadeeth of Anas ibn Maalik al-Ka’bi, according to which the Messenger of Allaah (peace and blessings of Allaah be upon him) said: ‘Allaah has exempted the traveller from the obligation of fasting and has reduced the prayer by half for him, and has exempted pregnant and breastfeeding women from the obligation of fasting.” (Narrated by the five).
End quote. From Tuhfat al-Ikhwaan bi Ajwabah Muhimmah tata’alliq bi Arkaan al-Islam, p. 171
And Allaah knows best.