“Rate” of anything doesn’t tell you how many actually are in a country at a particular time.

I normally work with divorce rate. Divorce rate is calculated as:

number of new divorces in a calendar year/number of marriages in a calendar year

Currently, that rate stands at about 48% or so for the US. There’s virtually nothing you can do with this number to estimate the number of divorced people currently living in the US (since your math skills aren’t good, you’ll just have to trust me on this.) It’s just a snapshot of one number compared to another number, and doesn’t account for the number of divorcee’s getting married that contribute to the denominator, nor does it factor in things that might make the number of marriages lower, like an increase in unmarried couples.

I believe that the rate of the birth defect you are looking at is calculated similarly:

(incidences of that birth defect in a year)/(total live births in a year)

Demographic “rates” aren’t the same rates that can be used in math to determine distance or velocity, for example. They aren’t predictive.

I suppose you could make some sort of estimate with enough variables, but rate alone wouldn’t be enough.