Manusmriti

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युग्मासु पुत्रा जायन्ते स्त्रियोऽयुग्मासु रात्रिषु । तस्माद् युग्मासु पुत्रार्थी संविशेदार्तवे स्त्रियम् ॥ ४८ ॥

sanskrit

On the even nights sons are conceived and daughters on the uneven ones; hence a man who desires to have sons should approach his wife in due season on the even (nights).

english translation

yugmAsu putrA jAyante striyo'yugmAsu rAtriSu | tasmAd yugmAsu putrArthI saMvizedArtave striyam || 48 ||

hk transliteration by Sanscript