I am using a difference-in-difference strategy with fixed effects -xtreg...,fe robust-. I am using as my treatment group mothers whose youngest child is younger than one.

I am investigating a few control groups. Firstly, mothers of older children and secondly childless mothers.

I define the variable treat as a dummy which takes the value of 1 for the treatment group and the value of 0 for my control group. I define post as my post reform period and I also control for some time-invariant factors. My dependent variable is wages.

When using xtreg with the first control group, nothing gets omitted, but when I use the second one then my treatment variable gets omitted because of collinearity.

I have tried to see if the number of childless mothers changes over time and it does so I don't understand why it gets omitted in the second case. I also have tried using no controls and I still get the same situation.

Any help is greatly appreciated