Manava Dharmashastra 3.49

3.49

śukraṃ vīryaṃ puruṣasya retaḥ, striyāḥ śoṇitam | uktaṃ bhagavatā vasiṣṭhena ( a0 15 sū0 1) “śukraśoṇitasambhavaḥ puruṣa” iti |
strībījādadhike pumbīje'yugmāsvapi putro jāyate, yugmāsvapi strībījasyādhikye kanyaiva | ayugmāsvapi rātriṣu putrārthino gamanānuṣṭhānārthametat | yadā paripuṣṭamātmānaṃ vṛṣyāhārayogena samadhikavīryaṃ manyeta striyāśca kathaṃcidapacayaṃ tadā putrārthī gacchedvityupadiṣṭaṃ bhavati |
ādhikyaṃ cātra na parimāṇataḥ, kiṃ tarhi sārataḥ |
same'pumān miśrīkṛte puṃstriyai | apumān napuṃsakamiti kecit | anye sāmya iti paṭhanti | ubhayoḥ sāmye'pumāneva |
puṃstriyau vā | garbhādhānyāṃ yadā vāyurdvavarūpatvātsaṃsṛṣṭe śukraśoṇite samaṃ vibhajeta ekatra bhāgamanyatra tāvadeva tadā yamau jāyete | tatra same vibhāge'pi strībījādhikye strī, pumbījādhikye pumān |

This statement is meant to lead the man seeking for a son to have intercourse with his wife on the uneven days also; the sense of the instruction being that—when the man finds that by the use of aphrodisiacs and strengthening food he has become vigorous in his virility, and that his wife has, for some reason or other, become weak, then he should have intercourse with her, when desirous of getting a son.

The ‘excess’ meant here is not that in quantity, but that in virility.

When the two are equal, there in burn either a non-male, or a boy and a girl, together. ‘Non-male’ stands for the hermaphrodite, according to some people.

Some people read ‘sāmye;’ and it means that ‘in case of equality of both, a non-male is born.’

‘Or a boy and a girl’—When the wind in the womb stirs up the mixed semen and ovula and divides it into two equal parts—a small quantity in -one part, and an equal quantity in another part of the womb,—then twins are born; and in those two equal parts also, in that part where the woman’s seed happens to be in excess the girl is born, while in the other part, where the male’s seed is in excess, the boy is born.