In this case, assuming a woman had no pre-existing veset kavua (extablished veset), she observes only the interval, based on her last two niddah cycles.
If she is age 53 or older, the rules are even more lenient, to reflect the body’s transition. In that case, she does not observe even this interval unless it is either ninety days or less or becomes established by recurring exactly three consecutive times.
We hope this is clear. Please be in touch with any further questions.