Tradition and history — not medical advice
Rest, work and intimacy after birth, in Suśruta
The same verse that prescribes oil and washing also names three things the text says a new mother should give up for a while: sex, physical labour, and giving in to anger.
What the texts say
Suśruta Saṃhitā — Śārīra-sthāna 10.16
A mother, after parturition, should forego (for a considerable time) sexual intercourse, physical labour and indulgence in irascible emotions.
Kaviraj Kunja Lal Bhishagratna, An English Translation of the Sushruta Samhita, Based on Original Sanskrit Text, Volume II (Nidāna-sthāna, Śārīra-sthāna, Chikitsā-sthāna and Kalpa-sthāna) (1911). Public domain.
Suśruta Saṃhitā — Śārīra-sthāna 10.17 ("memorable verses")
Any disease acquired by a newly delivered mother (Sūtikā) by her injudicious conduct of life soon lapses into one of a difficult type (hard to cure); and it becomes incurable if it be due to too much fasting.
Kaviraj Kunja Lal Bhishagratna, An English Translation of the Sushruta Samhita, Based on Original Sanskrit Text, Volume II (Nidāna-sthāna, Śārīra-sthāna, Chikitsā-sthāna and Kalpa-sthāna) (1911). Public domain.
What modern guidance says
There are no rules about when to start having sex again after you have given birth. You'll probably feel sore as well as tired after your baby is born, so don't rush into it.
NHS (reviewed 2024-02-07)
Speak to your doctor or midwife
If you're not sure when it's fine to return to exercise, lifting or work after birth — especially after a caesarean or a difficult delivery — ask your doctor or midwife rather than following a fixed rule from any source, old or new.