Niddah 15; Assumptions about a Change in Status.

Does it matter if a woman immerses in a mikvah?  Today's daf looks at safek, what is unknown, and vadai, what is reality.  If a woman returns from mikvah, should her husband assume that she has changed status from niddah to ritually pure?  We are told the story of a maidservant who drops a non-viable infant into a pit.  When a priest looks to see whether the child is male or female (to determine other rituals), he could have become ritually impure.  The rabbis say that he is ritually pure, since the infant likely was removed by a weasel or another predatory animal.