This is what every women will ask once a month.  I would say, it is a matter of personal preference and you should listen to your body. If you feel tired on the first or second day of the period, take sometime off.  For women who do choose to practice, it is suggested that they avoid inverted poses like headstand or shoulder stand, abdominal strengtheners, extended holding of any pose, or energizing breaths like kapalabhati. The issue is that these practices might interfere with the downward of the menstrual flow or cause discomfort to the body.

As what I have observed in my body ever since I started Yoga practice, the rate of the flow does increase on the first two days and the subsequent days is normal. It may stop at day 4 or day 5 unlike previously, it was dragged thru the whole week.

As for me, I will resumed class at day 4 or 5 when the flow is hardly even there.  I do feel a little embarrassed  and awkward to be singled out during an asana.  You know what I mean when the whole class know that you are having your menses.  What make it worst, when the words came out from the mouth of a male instructor. *yikes*

Namaste, everyone!