Understanding Society User Support

Dear Understanding Society team,

We're currently using wave 11 data and want to distinguish birth and non-birth parents. My current code is based on variables 'mnspid' (Nat/step/adopt mother: PIDP) and 'mnpid' (Natural mother: PIDP). Based on the variable description, I thought 'mnspid' indicates all mothers' IDs, no matter whether they're birth mothers or not, and 'mnpid' indicates only birth mothers' IDs. So I compared 'mnpid' to 'mnspid' and labeled those who have overlapped IDs as birth mothers, and those non-overlapped as non-birth mothers. However, the results show there are only 4 non-birth mothers in my dataset, which involves around 5,000 households. I don't think this result is reasonable. Therefore I'm wondering if you can kindly help me to clarify my questions regarding the variables?

(1) I noticed in the description 'mnpid' indicates it refers to pidp of 'natural mother.' Does that refer to 'birth mother'?

(2) If my approach is problematic, is there a better way to distinguish birth and non-birth mothers in Understanding Society?

Thanks in advance for your time, and I look forward to your reply.

Best,
Yushi