The Compendium of Caraka 8.9.62

8.9.62

strīṇāmārtavakāle tu pratikarma tadācaret/
garbhāsanā sukhaṃ snehaṃ tadā++ādatte hyapāvṛtā//

For women, however, one should perform that treatment at the time of menstruation;
for then, her womb being receptive, she easily takes up the unctuous substance, since it is open.