We are sorry to hear of your miscarriage.
If you had bleeding prior to the procedure, then that could have already made you niddah, based on the usual criteria for becoming niddah. If you did not previously have bleeding, then the D&C procedure will make you niddah.
In either case, you may perform a hefsek taharah as early as day five (with day one being the day on which you became niddah), but you may not immerse until fourteen days have passed from the D&C, with the day of the procedure counted as the first day.
The exception would be if it was determined that the fetus was male, in which case you could immerse as soon as you complete your clean days.
Please get back to us with any further questions. We wish you a refuah sheleimah, refuat hanefesh and refuat haguf.