Answer
Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
The pregnant woman must stand if she is able. If standing is difficult or she has a threatened miscarriage and must sit or lie down, she can do so.
Shedding more light on the question in point, we would like to cite for you the following fatwa published on www.islamqa.info:
The basic principle concerning the prayer of one who is sick is that he/she should do whatever he/she is able to of the essential parts of the prayer, and he/she does not have to do what he/she is unable to do. This is indicated by a great deal of evidence from the Qur’an and Sunnah. Allah says: “So keep your duty to Allah and fear Him as much as you can.” (At-Taghabun 64:16)
“Allah tasks not a soul beyond its scope.” (Al-Baqarah 2:286)
And the Prophet (peace and blessings be upon him) said: “If I command you to do a thing, then do as much of it as you can.” (Al-Bukhari and Muslim)
It was narrated that `Imran ibn Al-Husayn (may Allah be pleased with him) said: I was suffering from hemorrhoids and I asked the Prophet (peace and blessings be upon him) about praying. He said, “Pray standing, and if you cannot, then sitting down, and if you cannot, then lying on your side.” (Al-Bukhari)
Based on the above, if your wife is able to pray standing up, then she has to stand. Then if she becomes unable to stand or it is too difficult for her to stand, then she can sit down while praying.
It is permissible to sit on a chair or on the ground, depending on what one is able to do and what is easy for him/her. But it is better to sit on the ground, because the Sunnah is for a person to sit cross-legged in the place where one would stand and bow, and this is not easy to do on a chair.
Sheikh Ibn `Uthaymeen said: If a person cannot pray standing, he/she should pray sitting, but it is better to sit cross-legged in the place of standing and bowing.
From his essay, Taharat al-Marid wa Silatuhu, he states,
This sitting cross-legged is not obligatory; he/she may sit however he/she wants because the Prophet (peace and blessings be upon him) said: “If he cannot then sitting,” and he did not explain how one should sit.
If it is too difficult for your wife to prostrate and bow, she can lean forward and make the leaning for sujud deeper than that for ruku`.
If she can stand, then lean forward for ruku` while standing, and lean forward for sujud while sitting, then she should do that, because standing is closer to bowing than sitting, and sitting is closer to sujud than standing.
Sheikh Ibn Baaz said:
Whoever is able to stand but is unable to bow or prostrate is not relieved of the obligation to stand; rather he should pray standing up, then lean forward for ruku` (i.e., while standing) then sit and lean forward for sujud… and he should make leaning for sujud deeper than that for ruku`. If he is only unable to prostrate, then he should do ruku` and then lean forward for sujud.
If during the prayer the sick person becomes able to do something that he/she was unable to do, such as standing, sitting, bowing or prostrating, he/she should start to do that and continue from whatever he/she has already completed of his/her prayer.
From his essay Ahkam Salat al-Marid wa Taharatihi, Sheikh Ibn `Uthaymeen said,
“Whoever is not able to bow should lean forward while standing, and whoever is not able to prostrate should lean forward while sitting.”
Allah Almighty knows best.