What is the ruling regarding the prayers performed by the questioner while lying on her back, even though she was able to pray on her side? Is it permissible for her to pray while lying down if she is able to stand and sit, in order to avoid the risk of miscarriage? And must she make up those prayers?

If it is highly probable that performing Salah standing or sitting will lead to a miscarriage, then it is permissible to pray while lying down. Praying while lying down is done by reclining on one's side (the right side is preferred) while facing the Qibla and signaling for Ruku' (bowing) and Sujud (prostration) with one's head, with the Sujud being lower than the Ruku'. It is also permissible to lie on one's back with the feet towards the Qibla, though this is disliked by the Hanbalis due to disagreement regarding its validity. If the worshipper is unable to signal with their head, they may signal with their eyes and intend with their heart. Praying on one's back is valid. If a woman prayed in an impermissible manner due to ignorance of the ruling, the more cautious approach is to make up the prayers by estimation and probable conjecture.