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>>Italy has around 9 births per 1000 people. The numbers are spot on.

>>The US has around 3.9 million births per year and 286 million people. That is 1 per 73 residents, or around 13.6 per thousand.

>>US Catholics have 1.05 million infant baptism per year and 65 million people, or 16 per thousand.>>

Math checks out to me. But note: 16 per 1000 means population stability if this rate holds up across generations, and you people reach 62 years old. (1000/16=62) So we should be growing. OTOH, there is a bulge in the fertile years due to the baby boom, so the 16 is inflated... so we're really just about breaking even.

Now, consider Italy. At 9 births per 1,000, they'd have to live 111 years to have enough babies! But a longer life expectancy doesn't mean longer fertility, so even that's no use. They need a fertility rate of 14 per 1,000 to maintain the population. At only 9 per 1,000, they are having only 1.3 babies per fertile family, not 2.1, so their population is collapsing.

BUT... I just ran the numbers myself.. At that rate of collapse, it would take 700 years, not 300 to reach a population of only 3. GEEZ, isn't there any research I don't have to fact-check myself!

OTOH, there is a bulge in the fertile years due to the baby boom, so the 16 is inflated... so we're really just about breaking even.

That bulge has essentially passed, while the number of Baptisms has held up. The peak birth years of Baby Boom Catholics were 1955-1965. Only the last couple of years of these folks could possibly be considered still fertile.

For purposes of simple evaluation, its enough to consider a woman's fertile years from 25 to 40, when she is most likely to be married and having children.