A pidyon haben (redemption of the firstborn son) is not dependent on whether a woman went to the mikveh before conceiving.

If a woman miscarried within forty days after conception (usually forty days after going to the mikveh, in halachic terms), her next child is considered her firstborn, and, if a boy, should have a pidyon haben.

If the miscarriage occurred more than 40 days after conception, there will be no pidyon haben unless there was no "rikum" (tissue). If there is doubt as to whether or not there was tissue in the miscarriage, then a pidyon is required without a bracha and with the kohen having in mind to return the money. We suggest discussing this point with a local rabbi if it becomes relevant.