Why were births never registered? Was that during a particular period, so one particular age group are most likely missing certificates, or is it spread across generations?
I'm in the UK, and have my certificate.
It is spread across generations. The charity group assisting with registering births, even those which occurred decades ago, say they get a warm glow when a whole class of children proudly holds up their certificates. Factors include isolation, poor literacy, and English as a second language. It is particularly common in the indgenous population.