BIDX reports the index number of the child in the mother's birth history, from 1 to n, where the nth birth is the mother's first birth, and 1 is the most recently born child. Up to 20 births could be reported by a woman. After asking "Now I would like to record the names of all your births, whether still alive or not, with the first one you had," the interviewer recorded each child's name in turn on successively numbered lines printed in the "Reproduction" section of the individual woman's questionnaire.
Combined with CASEID, BIDX uniquely identifies children within a sample. Combined with IDHSPID, BIDX uniquely identifies children across samples. Using these variables, data users can link IPUMS-DHS data to original DHS child-level and birth-level datasets.