The Compendium of Bheḷa 4.5.6

4.5.6

strīṇāṃ khalu śoṇitaṃ śarīraṃ śoṣayati tasmānna tāḥ rajaḥ paśyanti | paripūrṇadhātuśarīrās tu yadā bhavanti tadā vivekajalalohitaṃ māse māse prativedayanti | pratigatapraveśaṃ ca tatpunarmāsena samāgacchatyārtavam ||

Indeed, blood dries up the body of women; therefore they do not see the menses [during pregnancy]. But when their bodies have full tissue-elements, then month after month they discharge the separate fluid-blood. And that menstrual blood, its entrance having receded, returns in a month.