The Law Code of Yājñavalkya 1.3.125

1.3.125

evam uktena prakāreṇa striyaṃ gacchan kṣāmāṃ gacchet | kṣāmatā ca tasmin kale rajasvalāvratenaiva bhavati | atha cen na bhavati tadā kartavyā kṣāmatā putrotpattyartham alpāsnigdhabhojanādinā,

Approaching a woman thus in the manner described, he should approach her [when she is] slender. And slenderness at that time arises precisely from the observance of the menstruating woman. If it does not arise, then slenderness should be brought about for the sake of producing a son through small and unctuous food and the like,