Individuals whose birth was not recorded when they were born may have filed for a delayed birth certificate when they reached the age of majority, realized they needed a certificate and did not have one, applied for a social security number, or a combination of reasons. The majority of delayed birth certificates in the United States were filed for individuals who lived into the 1930s and beyond.
How the delayed certificates are filed depends on the location. Some kept an entire separate set of certificates that were filed some time after the fact. Some areas just filed the delayed certificates with the original certificates filed when the individual was born. Some changed their procedures over time.
Find out how areas in which you have an interest filed these records.