The Compendium of Caraka 4.4.7

4.4.7

yayā cānupūrvyā 'abhinirvartate kukṣau tāṃ vyākhyāsyāmaḥ---gate purāṇe rajasi nave cānasthite śuddhasnātāṃ striyamavyāpannayoniśoṇitagarbhāśayāmṛtumatīmācakṣmahe/

And the order by which it develops in the womb we shall explain: When the old menstrual blood has departed and the new has not yet settled, a woman who has bathed clean, whose vagina, menstrual blood, and uterus are free from disorder, we declare to be in her fertile season.