I’m going to stick my neck out here. I suspect (but don’t know for certain) that the birth certificates available on FamilySearch now include all of the Chicago records that were available when the records were microfilmed.
My guess is that if a researcher can’t find a birth record online, it’s for one of two reasons:
1) The birth wasn’t reported so there’s no birth certificate to be found.
2) The name is misspelled in the index.
One of the best ways to check for both possibilities up through 1915 is to use the Chicago birth registers which are also online.
A little bit of background. It seems reasonable to me that the birth registers were created to log birth certificates that were returned to the county. The entries in all but the earliest books group names together by the first letter of the surname for each month. For example, all the “G” entries for April 1901 would be together, followed by all the “G” entries for May 1901, etc. The entries aren’t alphabetical. They’re in certificate number order which makes me think that the numbers were assigned as the records were recorded in the books.
So, because of the arrangement, it’s possible to use the registers as an alternate index if you know the month and year of birth or if you have a good deal of patience . . .
Here’s how to do this at FamilySearch using an example of an “S” birth from January 1891:
1) Navigate to the search page for “Illinois, Cook County Birth Registers, 1871-1915.
2) Type a common name with the same letter of the name you’re looking for in the surname box and type the year of birth in the year ranges boxes. In this case, I'm looking for an "S" birth, so I’ll use “Smith." For surnames beginning with other letters, you may have to try a few different names to be able to find a register in the ballpark of the month/year/name you're looking for.
3) Browse to find a birth from the same month you’re looking for. In this case, “Henry Smith” born January 1891 will work.
4) Click on the name, then click on the small thumbnail in the upper right-hand corner to view the register image.
5) Use the arrow buttons to navigate back to the beginning of the “S” section for January 1891. The month is usually on top of the left-hand page; the year is on top of the right-hand page. Each page has fifty entries and in my experience, even common letters of the alphabet have just a few pages to check.
6) View each register page looking for a match. I often look at the surname, the birth date, the mother’s name, and/or the father’s name.
A few tips on using the birth registers:
1) The catalog entries in the Family History Library Catalog can help you figure out the arrangement of the register entries. For example, in this case, the December 1890 “S” births came just before the January 1891 S” births. For some years, they might be in different books.
2) The register entries cross two pages.
3) For some of the later years, you might find stillbirths or infants who died soon after birth listed at the end of each section.
4) If you find a matching register entry, there should be a matching birth certificate and if you’re using film, you can use the register number to find the certificate. Most certificate numbers have five digits but you might see just two by the names on the page. Look at the very top line to find the other digits.
5) If you find a matching entry and want to locate the corresponding record on FamilySearch but haven't been able to pull it up there before searching the name, try searching for other names on the register page. If you can find a close certificate, you should be able to easily navigate to the right one.
If the birth certificates were assigned numbers when they were entered in to the registers then I think it’s unlikely that many were missed. If a certificate didn’t have a number, it couldn’t be filed away. And as long as a birth was recorded under the proper first letter, then it should be possible to pick out a match in the register even if the name is badly misspelled. Based on these two assumptions, if you don’t find a matching entry in the birth register, then it’s very likely there is no birth certificate to be found on FamilySearch.
If that turns out to be the case, there are a number of things you can do. Here are some ideas:
1) Look for a delayed birth certificate
2) Look for a baptismal record
3) Look for a Social Security application
4) Check for a passport application
5) Check for a school record