I don't quite see how why this specificity is neccessary.

Not that those women with very painful periods don't require leave as everyone who is in (unmanageable) pain - but could this not be handled just as any other kind of chronic/recurring pain issue is handled already? In particular as I understand this law does not mean that any women can just state they're in pain and go home. Just like with any pain issue, you have to see a doctor to grant you sick leave.

So what makes "uterus pain" special compared to migraines, stomach pain, pain in your limbs, etc.? Too painful to work is too painful to work, no matter where.