In return for Lord Indra’s benediction that they would be able to enjoy lusty desires continuously, even during pregnancy for as long as sex is not injurious to the embryo, women accepted one fourth of the sinful reactions. As a result of those reactions, women manifest the signs of menstruation every month.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
तेषां निर्यासरूपेण ब्रह्महत्या प्रदृश्यते । शश्वत्कामवरेणांहस्तुरीयं जगृहुः स्त्रियः
teṣāṃ niryāsarūpeṇa brahmahatyā pradṛśyate | śaśvatkāmavareṇāṃhasturīyaṃ jagṛhuḥ striyaḥ
SB 6.9.9