Unintentional miscalculation of menses is not a sin. If a woman thinks that she has become pure and starts praying and fasting, then she realises that she is still menstruating, then she has to stop praying and fasting until she becomes pure, and she should make up the obligatory fasts that she observed during that time.

Whoever does not fast some days of Ramadan has to make up those days before the next Ramadan comes. For more, please see the detailed answer.

It is permissible for a pregnant woman not to fast if she fears some harm which she thinks will most likely affect her and/or her baby.

The correct scholarly view is that if a woman miscarries a foetus that was fully formed, then she should stop praying and fasting because this is postpartum bleeding. If the foetus was not fully formed, then her blood is irregular bleeding and she should not stop praying and fasting.

If a Muslim misses fasting for many years, he/she has to ask Allah for forgiveness and repent to Him, and has to fast the days that he/she missed and feed one poor person for each day. If he/she is poor and cannot feed others, then fasting and repentance is sufficient and he/she does not have to feed others.

There is no need to do ghusl because of this discharge, and it does not affect the fast. With regard to prayer, wudu must be done for each prayer after the time for the prayer has begun, if the flow is continual.