Birth Mother in Lesbian Couples CPS

Hi, student researcher and first time poster here:

I’m currently working with lesbian couples in the IPUMS CPS data for 2000 to 2020, and I’m curious to see which woman in the relationship actually gave birth to any given child in the household. I have three questions relating to this task:

  1. I’ve thought about looking at MOMLOC and MOMLOC2 variables to determine this; but the MOM2RULE documentation says that the “second mother” is often simply the woman who is not the householder, so I believe this won’t tell me about birth per se?

  2. The self reported “Mother’s line number variable” PELNMOM seems promising, but it is not available for about 33% of the children in my data. Might it be the case that PELNMOM, when defined, usually points to the birth mother?

  3. Are there any other recommended variables or techniques to find which woman in a same-sex couple gave birth to the household children?

Thank you immensely for any help offered!!

EDIT: I noticed the PELNMOM is actually defined for all children in my sample. Using the “Mother’s relationship to child” PEMOMTYP variable, I can see whether the woman listed by PELNMOM is called a biological mother or step or adopted. If PEMOMTYP says biological, does this mean she is the birth mother? And if PEMOMTYP lists adopted, is there a way to tell if said adoption was an in-union adoption or not?

Overall, I would not recommend using the IPUMS family interrelationship variables (MOMLOC, MOMLOC2) or the self-reported family interrelationship variables in the original CPS data without further exploration or supplementing the information. However, the CPS fertility supplement may be helpful. Details on each of your points are below.

  1. The algorithm for IPUMS family interrelationship variables is based on social (i.e., not biological) relationships. I would not assume that the person identified via MOMLOC is more likely to be the birth mother; this is simply the person who was assigned to the child as a parent first (e.g., if they are the householder, they will be assigned via MOMLOC while their spouse/partner will be assigned via MOMLOC2. A more complicated example would be a householder residing with a married sister, a married sister-in-law, and a nephew; the nephew would likely be attached to the married sister first then the sister-in-law. Neither tells you anything about who is more likely to be the birth mother).
  2. PELNMOM/PEMOMTYP were first collected in 2007 and is available for the 2007-2018 ASEC and the 2007-2019 basic monthly data. Beginning with the 2019 ASEC and 2020 basic monthly data; CPS instead offers first and second parent variables (PELNPAR1/PEPAR1TYP and PELNPAR2/PEPAR2TYP) rather than the sex-specific PELNMOM/PELNDAD variables. You may be interested in the code at the very bottom of this page about the IPUMS USA family interrelationship variables, which has information on how to attach the characteristics of another family member as a new variable on a person record. It isn’t obvious to me how the non-birthing mother would be categorized by the PEMOMTYP variable. I wouldn’t use this without doing a bit more digging. You might contact the Census Bureau ( with a question about these variables for your application.
  3. You may be interested in the CPS Fertility Supplement (either on its own or in conjunction with the family interrelationship variables); for example, the variable FREVER reports the number of live births a woman has had.
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Thank you very much for your detailed response!

Unfortunately the fertility supplement is administered too infrequently to perform the kind of analysis I had planned, but it was a very good idea. I will reach out to the Census; thank you again for the help

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