What leniencies are available for women who ovulate before mikvah night (due to shorter than average cycles), and therefore most likely cannot get pregnant while following the laws of taharas hamishpacha?
The only one I am aware of is counting starting to start counting the 7 days after 4 days from the beginning of the period (if indeed the period only lasts 4 days or less), so the total time between period onset and immersion is 11 days instead of 12 days. However, this shift of one day is not always enough to ensure that ovulation occurs after immersion.
I've heard of a leniency that involves counting 4 (or 5) days of the period starting from the last intercourse before the period began, rather than from the actual period onset. However, I was told "we don't rely on this." Is this leniency acceptable to some poskim? Are there other leniencies in this situation?
(I am aware that some women take hormones to delay ovulation in this circumstance, but that is not an option for everyone.)