Ali bin Abd al-Aziz (may Allah have mercy on him) informed us that Abu Ubayd (may Allah have mercy on him) said that al-Asma'i (may Allah have mercy on him) and others said: "Aqra'at al-mar'ah" is said when the time of her menstruation draws near, and it is also said when the time of her purity draws near. Abu Ubayd (may Allah have mercy on him) said: The root of aqra'a is the arrival of the time of something. As a poet praises a man for a military expedition:
'Muwarrathatan malan wa fi al-dhikri rif'atan, lima da'a fiha min quru'i nisa'ika'
"In the pursuit and gain of wealth and the elevation of name, the quru' of their women were lost because of it." So here quru' means the periods of purity, because intercourse with women takes place only during purity.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15401