For reasons that are not entirely apparent, Lillian Apgar's birth certificate was recorded on 3 March 1931, one day after her twenty-first birthday. Her sister's was recorded a few days later, but has the wrong date of birth. These certificates were recorded with the 1910 births and not in a separate series of records.
These certificates and the certificate for a younger brother of these two sisters are analyzed in an article on our site.
What was interesting in this case of Chicago birth certificates was that of the four children the parents had, one daughter's record was never recorded.
Unfortunately that missing daughter is my wife's grandmother.