Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 7 · Samskara Muhurtas
संस्कारमुहूर्ताः
आद्या निशश्चतस्रस्तु त्याज्या ह्यपि समाः परैः ।ओजराश्यंशगे चंद्रे लग्ने पुंग्रहवीक्षिते ॥ ३१८ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 318.
The first four nights must be left out. Others say the even nights should be left out too. Choose the Moon in an odd sign and an odd navamsha (one-ninth part of a sign). The lagna (rising sign) should be seen by male planets.
Simple meaning: Skip the first four nights after the period. Prefer the Moon in an odd sign and male planets looking at the lagna.
उपवीती युग्मतिथावनग्नः कामयेत्स्त्रियम् ।
A man who wants a son should keep to even moon days and avoid four stars.
प्रसिद्धे प्रथमे गर्भे तृतीये वा द्वितीयके ।
The rite for a son is done in the 2nd or 3rd month of the first pregnancy.
चतुर्थे मासि षष्ठे वाप्यष्टमे वा तदीश्वरे ।
Choose the 4th, 6th or 8th month, when the Moon and the stars suit both parents.