The Compendium of Kāśyapa 5.9.17

5.9.17

niruṇaddhyārtavaṃ tatra māsikaṃ saṃcinoti ca
rakte ca saṃsthite nārī garbhiṇyasmīti manyate

There it obstructs the menstrual flow and accumulates month by month; / and when the blood remains accumulated, the woman thinks, “I am pregnant.”