How is it possible to have mothers with no children in the US Census 1990?

When I attach as a characteristic “the children ever born of mother” I find some mothers having zero children. How is this possible?

The “children ever born” variable (CHBORN) does not include adopted children or stepchildren. In contrast, the Attach Characteristics option identifies mothers using MOMLOC, which includes stepmothers and adoptive parents. Nearly 98% of these cases in the 1990 Census where a child’s mother never gave birth have a STEPMOM value greater than 0, indicating the child was likely a stepchild or adopted. Since many stepmothers and adoptive mothers cannot be identified with the information available, STEPMOM always under-represents their actual number in the population.

Hope this helps.