Today I'll focus on the Chicago Delayed Birth Index that's newly available in digital format through the FamilySearch Catalog. You can view the index images if you visit a family history center or FamilySearch affiliate library; you won't be able to view the images from home. And, you may be able to find the matching records online, too.
First, a little bit of background.
For those of you who are unfamiliar with delayed birth registration, it was a way of creating a civil birth record, sometimes long after the birth, when an original wasn't filed at the time of the event. It's particularly relevant to Chicago research because many early births went unreported.
Many of the entries in this index are for records that were recorded in the 1940s. Why? One reason might have been that people who were going to work for the war effort needed to be able to prove their citizenship. 
So, when should you use this index?
Checking this index is a good next step when you've searched the "regular" birth indexes and come up empty-handed. Why? Because an unsuccessful search for a birth certificate can mean two things--the birth wasn't reported or the name, for whatever reason, just isn't popping out of the index. If you find a match in this delayed birth index, then it's likely the birth wasn't reported at the time of the event and you can feel comfortable giving up the search for a record that likely doesn't exist.
And what should you know about searching it?
Just one thing. Surnames aren't listed in strict alphabetical order and it isn't sorted by given name. If you are search "Smith," you will have to look through many pages. The good news is years appear to cluster and you can use that to quickly skim through irrelevant entries. Take a good look at this example page to see what I mean.
In the past, I've suggested people contact the Cook County clerk's office to obtain copies of these record. However, it appears that at least some of the matches are included in the newly-digitized Cook County records that are available on at FamilySearch and their online index may duplicate this delayed birth index meaning--you may not have to use this index at all! For more information and a tutorial on how to find the records on FamilySearch, please watch the video at the top of the post.
 Alfred A. Worzala, "Your Social Security: How to Prove Citizenship," Chicago Tribune, 15 November 1970, sec. 5, p. 11, col. 1; digital image, Chicago Tribune Archives (http://archives.chicagotribune.com/1970/11/15/page/145/article/your-social-security : accessed 13 September 2017).