Is prayer obligatory for a woman who experienced intermittent bleeding for a month after the miscarriage of one of her twins, given that doctors attributed this bleeding to retained fetal tissue or partial placental abruption, and she did not pray during that period?

The preponderant opinion is that a pregnant woman does not menstruate, based on the Prophet's (peace be upon him) statement regarding the captives of Awṭās: "A pregnant woman should not be approached until she gives birth, nor a non-pregnant woman until she has menstruated once." He made menstruation a sign of the womb's emptiness. Therefore, any blood a pregnant woman sees is considered irregular bleeding (istihada). The blood you saw is not considered nifas (postnatal bleeding) because the miscarriage did not show the formation of a human being. Consequently, you were required to perform what is incumbent upon a woman experiencing istihada: performing ablution for each prayer after its time has begun. You must also make up the prayers you missed.