If you believe that everyone currently walking the earth has a birth certificate, you would be wrong. Moreover, to get a passing sense of the magnitude of your error, you would need to look no further than to the world's largest democracy, home to over 1.4 billion people, many of whom lack birth certificates: India.
Prior to enactment of the "Registration of Births and Deaths Act" in 1969, births and deaths were not required to be registered under Indian law. Even though every Indian national born after March of 1970 should, in theory, have a birth certificate, the law does not match reality.
As recently as 2007, 37 years after birth certificates became mandatory in India, 25% of annual births had not been registered with the Indian government according to a news article. Now, no less than 54 years after the Act's enactment in 1969, news broke that the Indian Parliament may amend the Act for the very first time, causing birth certificates to become more meaningful documents there. See Bill in Parliament for multiple use of birth certificate. In India, people without birth certificates typically use other documents, such as school records, to prove their identities. As a result, birth certificates can almost seem like a luxury there.
However, swing around to the other side of the planet, the United States, and norms there are completely different. With birth certificates being commonplace in the U.S., the lack of a birth certificate creates real headaches for Indian nationals who are trying to become Lawful Permanent Residents of the U.S. This is because the U.S. controls the volume of immigration by imposing annual quotas of visas on every country based on where the immigrant was born. As a result, the U.S. routinely requires and uses birth certificates in order to determine against which country's allotment of visas to "charge" an individual immigrant's visa. Without knowing against which country to "charge" the visa, a person cannot immigrate to the U.S.
For any Indian who does not have a birth certificate despite being born after April 1, 1970, a "Certificate of Non-Availability" will have to be obtained from the local authorities with jurisdiction over his place of birth and given to U.S. Citizenship & Immigration Services ("USCIS"), along with secondary evidence listed below.
For persons born on or before April 1, 1970, USCIS will presume that a birth certificate is unavailable. However, that doesn't settle the issue. The immigrant would still need to provide USCIS with secondary evidence, for which acceptable documents are listed below. (This list is available in the DOS Reciprocity Schedule).
1. School-leaving Certificate (document provided to students when they cease attending a particular school, be it public or private),
2. Matriculation Certificate,
3. Certificate of Recognized Boards from the school last attended by the applicant (Boards are exams)
4. A notarized affidavit executed by either a parent, if living, or another close relative older than the applicant. This affidavit should clearly state the relationship between the deponent and the applicant, how well the deponent knows the applicant, the date and place of the applicant's birth, the names of both parents, and any other related facts. If the applicant has no living relatives that witnessed their birth, a self-attested affidavit detailing their knowledge of the facts of their birth may be accepted.
U.S. immigration lawyers deal with these types of issues all the time. Speak with a lawyer in order to develop the best strategy for your situation.