We are sorry to hear of your miscarriage.
Unless bleeding begins before the procedure, the procedure itself will make you niddah. Overall, the process of becoming tehorah is like the standard procedure following niddah, with one key difference. Since you were fifteen weeks along and the gender of the fetus is unknown, you may immerse no earlier than the night following the fourteenth day from the procedure. For example, if your D&E is on a Thursday during the day, you would be able to immerse as of Wednesday night, nearly two weeks later. In practice, though, bleeding may last longer.
Since staining is fairly common, we recommend reading up on the laws of stains (as by reading our articles on stains and toilet paper) for guidelines that may help you reach the mikveh. We also encourage you to be in touch with us if you have any difficulty getting through the clean days. For this cycle, you may also omit the moch dachuk.
Your veset calendar was reset by the pregnancy, so you have no onot to observe until after you next become niddah.
Hospital practice regarding fetal remains varies, though they are usually treated with respect. According to Halacha, they should indeed be buried if they have a recognizably human form, which is likely the case here. You (or someone else you entrust with the task) can clarify the hospital’s policy and contact the local chevra kadisha for assistance.
You may find it helpful to read our article on pregnancy loss. Please don’t hesitate to get back to us with any additional questions that arise.
We wish you a smooth procedure, and a refuah sheleimah — refuat hanefesh urfuat haguf.