Jatakaparijata 13.17

13.17

atha strīṇāmṛtukālaparimāṇam |
vibhāvarīṣoḍaśa bhāminīnāmṛtudramādyā ṛtukālamāhuḥ |
nādyāścatasro'tra niśekayogyāḥ parāśca yugmāḥ sutadāḥ praśastāḥ ||

Now, the duration of the fertile period of women:
They say that sixteen nights from the onset of the menses are the fertile period for women;
here, the first four are not fit for impregnation; the subsequent even nights bestow sons and are praised.