What is the ruling on the prayer of a pregnant woman who is unable to completely purify the anus and finds a trace after prayer?

The obligation in Istinja' (cleaning oneself after relieving oneself) is to pour water on the site of impurity until the area returns to its original rough texture. Overcoming the presumption of impurity's removal is sufficient. If you see something after prayer and it is probable that it exited after the prayer, then it is attributed to what came after the prayer. If you are certain that an impurity you could not remove remained, and you were unaware of its presence, then your prayer is valid. This is because avoiding impurity is a condition when one is aware and capable. Therefore, what you were unable to remove or were unaware of its existence does not affect the validity of the prayer. If you suffer from waswasa (whispers/obsessive doubts), leave it and there is no need for searching and inspecting. It is sufficient for you to perform Istinja' until you overcome the presumption of the impurity's removal.