IIRC, in the book, the wives are those who were married to the commanders pre-Gilead and supportive of the regime. Women are assigned a role depending on both their status and fertility:
Wives - married to commanders, supporters of Gilead regime, no 'sinful' blemish on their character. To act as the wife but have Martha's for cleaning and handmaids for childbearing. Fertility doesn't seem to determine their role/rank.
Aunts - the highest female status under wives. No 'sinful' behaviour in past, supporter of Gilead, unmarried and past fertile age
Marthas - unmarried women past fertile age, with no 'sinful' past. A lower status than aunt.
Handmaid - fertile women with a 'sinful' past
Unwomen - women who are not fertile and have a sinful past. These women would be sent to the colonies
Jezebels - women who are not fertile, have a sinful past, but are put as prostitutes rather than the colonies
Econowives - fertile women with no sinful past. These would be married to a low ranking man and expected to do all the roles of a wife, handmaid and martha.