Is it permissible for a pregnant woman to shorten her prayer or not perform it, based on a doctor's advice that prostration (sujud) and bowing (ruku') are dangerous for her due to the possibility of carrying twins, knowing that she does not feel tired during prayer? And is it permissible for her to pray while sitting, and what are the conditions for that?

It is not permissible for a Muslim to abandon prayer intentionally. Missing even one prayer until its time expires is a grave sin and a transgression greater than adultery, theft, drinking alcohol, and taking a life, by consensus. It is permissible for a pregnant woman to lighten the prayer by performing only the essential integral parts. However, it is not permissible for her to pray sitting unless a trustworthy and religious doctor informs her that standing would harm her or her fetus. It is preferable for her to sit cross-legged, though praying in any other sitting posture is permissible. She is required to perform bowing (ruku') and prostration (sujud) if she is able. Otherwise, she should gesture for them, making her prostration lower than her bowing.