I am a analysing data for a study that looks at risk factors for postpartum depression in a special population. I am using SAS studio for the analysis.

I am running logestic regression for each variable (risk factor) and all risk factors are binary variables.

Some of the risk factor variables have very few postives (for example I am looking at postpartum UTI as a possible risk factor and only 18 people out of 68 with the depression has it).

My questions is, is there a rule for the minimum number of positive observations for a variable to be considered for inclusion in the final regression model?

I have a small sample size (total: 300, depression: 68).

Thank you