The Vedas say that women menstruate as they took a portion of Indra's sin for killing a brahmin. Didn't women menstruate before this happened? How did they give birth before this happened?
In Rig Veda elimination of Vritra by Indra was mentioned .
According to the Rig Veda, Vritra kept the waters of the world captive until he was killed by Indra. Tvashtra fashioned the thunderbolt (Vajrayudha) for Indra. Thereafter, he defeated Danu, the mother of Vritra.
In Puranas it was narrated that Tvaṣṭā, created Vṛtra to exact his revenge. And, Indra eliminated even Vṛtra.
Rig Veda pre-dates Yajur veda and Puranas. So this episode of elimination of Viśvarūpa and Vritra of from Puranas might be an interpolated one.
There was no mention about killing of Viśvarūpa in Rig Veda. Further, in Rig Veda mention about Danu, mother of Vritra was made.
It indicates that women used to menstruate and procreate even before that episode.