How is the new fertility-related expense calculated for the past 10 years calculated?

If you incurred fertility related expenses in a prior year and had not included them as medical expenses in the year paid you will need to amend the applicable tax year for these additional expenses. They will be added on to any previously claimed or available medical expenses. You may have claimed an amount that was to low to be of benefit, but now with the additional expenses claim the total will generate a tax credit. Make sure that you are claiming the total of all expenses incurred during the tax year now including the Fertility Expenses. See below to learn how to amend a prior year tax return.

https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/change-your-return.html