Stuff like this, while done with good intentions, can threaten to entrench discrimination by having different sets of rules and expectations for people based on their gender. A better solution would be to give 12 extra holiday days per year to everyone, but make it known that it's acceptable to take them under those circumstances.
Is on birth control to supress having a period.
Has an IUD and is one of the 1 in 5 for whom this stops Having periods for a year.
Has had a hysterectomy.
Has gone through menopause.
Has another condition which causes them to not have periods.
do they have their 12 days ripped from them unceremoniously?
It seems like there are a lot of edge cases here which may provide additional areas to cry "sexism" because for the cases listed above they wont require the 12 extra days, yet without having intimate medical knowledge of your staff, you cannot differentite between one type of non-period having staff member and another (in the "other" case a male).
in any case I'm glad they've done it as a case study on what happens next.