Depends on the drug. Some illicit drugs can stay in your system for weeks after use, potentially causing harm to a new fetus. Consult with your OB doctor.
Yes and no. Some may be lucky with mild complications.Most may have withdrawal symptoms and other medical complications.They can have major psychological, emotional, neurologic, social issues.Why risk it!
Depends. Depends on what "right before" means. Pregnancy often diagnosed at 6-8 weeks post-conception, so you may have already been pregnant while using. The early weeks are very important in development. Best to do is stop, follow instructions carefully, and hope for the best. I know people whe are smart and successful whose mothers used drugs, but also seen many vegetative, sick "drug babies.".