Praise be to Allaah.
Firstly: it is not permissible for a Muslim to forego prayer no matter what the situation. But if he is sick, then he should pray according to what he is able to do. So he should pray standing up; if he cannot do that, then he should pray sitting, and if he cannot do that, then on his side. The evidence for that is:
1 – ‘Imraan ibn Husayn (may Allaah be pleased with him) who said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) said: ‘Pray standing, and if you cannot, then sitting, and if you cannot, then on your side.’” (narrated by al-Bukhaari, 1117)
2 – It was narrated from Jaabir (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said to a sick person who prayed using a pillow for rukoo’ and prostration “Pray on the ground if you can, and if you cannot, then tilt your head, and make your sujood lower than your rukoo’.” (Narrated by al-Bayhaqi with a strong (qawiy) isnaad, 4359. Classed as saheeh by al-Albaani in al-Saheeh, 323). See also Question no. 7522.
Secondly, although you say that you feel nauseous during pregnancy, this should not prevent you from doing the prayers; even if you feel nauseous when you pray, this does not invalidate your prayers.
Thirdly, can a woman join prayers if she finds it difficult to pray each prayer at its own time?
Examples of difficulty include travelling, sickness, rain, mud and intense cold wind.
But shortening the prayers is only permitted when travelling.
This woman who feels nauseous when praying will be rewarded in sha Allaah for striving to pray when she is in this difficult situation. Whatever she suffers of pain and exhaustion in her pregnancy will also expiate for her sins, because the Prophet (peace and blessings of Allaah be upon him) said: “No fatigue, no disease, no sorrow, no sadness, no hurt, no distress befalls a Muslim, not even the prick of a thorn, but Allaah expiates some of his sins for that.”
(Narrated by al-Bukhaari, al-Murda, 5210 – from Abu Hurayrah)
And Allaah knows best.
Sheikh Muhammed Salih Al-Munajjid