Is the cessation of bleeding after 18 days from a D&C procedure considered a state of purity (tahara) despite not seeing the usual signs of purity?

The blood you saw after the curettage operation is considered nifas (post-natal bleeding), as long as the fetus had developed human features. Therefore, it was obligatory for you to abstain from prayer and fasting while experiencing this blood. If the bleeding stopped completely after eighteen days and you observed dryness (al-jufūf), then you have become pure, and fasting and prayer become obligatory upon you. However, if the miscarriage did not show human features, then the blood is considered istihada (irregular vaginal bleeding), and you must observe what is required of a woman experiencing istihada, such as taking precautions and performing ablution for each prayer.