It is narrated from Imam Hasan, may Allah have mercy on him, regarding the divorced woman whose menstruation is not known (i.e., whether it comes or has stopped), he said: She will wait for a full year; if menstruation occurs, then fine, otherwise after one year she will observe the waiting period ('iddah) for three months. Then if menstruation occurs, then fine; if it does not occur, then her 'iddah is complete.