by ghostoffuffle

State offices were overwhelmed partially because most of them had only existed for a few decades; modern compulsory birth registration systems in the US were generally a Progressive-era reform designed to generate more precise infant mortality numbers. They were woefully incomplete in most states until the 1920s or early 1930s. Someone who was 25 years old in 1941 was born in 1916, when less than one-third of the American population lived in a state which registered 90 percent or more of each year’s births.20 By 1922, only 29 states met that 90 percent standard, which was set by the US Census Bureau in 1915 to encourage better state birth registration laws.

N.B. yellowoftops: although this focuses on those born in or before the 40's, I'm fairly certain I've read several accounts of rural folks after this time period never getting birth certs due to deliveries in a rural home setting using lay midwives or their own two mitts. So it's not necessarily a problem that will "die off" anytime soon.