How to determine county from county number in Texas birth index?

Question

The Texas birth index from 1956 gives a county number of 057 for a birth record I found, how can you determine what county that corresponds to, and more generally, how can I map any county number in Texas vital statistic records to its name?

I think the answer is Dallas County, but I'm having trouble finding an official Texas document that defines these county code numbers. I checked page one of the Microfiche, but no legend was there, it just starts in immediately with the index to the records.

The index is found on Ancestry.com and cites as:

Source Information
Ancestry.com. Texas Birth Index, 1903-1997 [database on-line].
              Provo, UT, USA: Ancestry.com Operations Inc, 2005.
Original data: Texas Birth Index, 1903-1997.
               Texas: Texas Department of State Health Services.
               Microfiche.
Description
  This database is an index to over 15 million births recorded in the State of 
  Texas, USA, between 1903 and 1997. Information available in this index 
  includes: child's name, sex, birth date, and birth county. Some records may
  also include: names of both parents.

Here is an image of the index record:

Answer 1

Since you are accessing the database Texas Birth Index, 1903-1997 on Ancestry.com, look the database up in the Card Catalog.

Under the section About Texas Birth Index, 1903-1997 there is a description of the database, which includes the county codes.

County 57 is Dallas County.

FamilySearch has a collection Texas, Birth Index, 1903-1997 which is described as:

Index of births from the Texas Department of State Health Services. Index provided by Ancestry.com.

This is probably the same data. When you search at FamilySearch, the detail pages cross-index the county codes for you and show you the name of the county, which is convenient, but could be subject to error. If you have access to Ancestry, you could cross-check your results by searching on both sites.

You can find more information about the database in the FamilySearch Research Wiki article Texas, Birth Index (FamilySearch Historical Records).

Ancestry's source information gives this for the original information:

Original data: Texas Birth Index, 1903-1997. Texas: Texas Department of State Health Services. Microfiche.

Try the state archives or state libraries to find a copy of the microfiche. The county code table is probably at the start of the fiche.

This entry for Texas birth indexes, 1903-1976 on WorldCat.org would probably have the same table in it. You can enter your zip code into WorldCat's "Find a Library" locator to find the library nearest you.

Even when you have access to a collection which is indexed and searchable by name, it's always a good idea to browse the images at the beginning and the end to see if there are pages explaining the format of the entries and the arrangement of the volumes. If the index makes use of county codes, the codes are often printed in a table in the front of the printed book or in an appendix at the end.

Ancestry isn't always diligent about making those images available to us, but you can't know what's there if you don't look. I often save the title pages of volumes to have a record of the publication data, plus pages of abbreviations and other front matter.

Answer 2

I found a source for these using Google to search "texas county code numbers".

They seem to come from the Texas Comptroller of Public Accounts and 057 matches Dallas county there.

The codes there match those from the Texas Department of State Health Services which makes them available as an Excel spreadsheet for download and says:

The county FIPS (Federal Information Processing Standards) code is assigned by the US Census Bureau.

They are American National Standards Institute (ANSI) Codes.