I would urge against using oregano oil during pregnancy or breast feeding. The woman's body should be in "high gear" during those times, and if you are following proper nutriion guidelines (primal or genotyping), and getting a great multiple mineral and vitamin formula, then you should be able to fight off infection. Also, remember one important reason for mothers milk is to pass antibodies and immune components to the child. One should not compromise this aspect by taking something as strong as oregano oil.
Since I am not a woman, thanks for asking for specificity! LOL!