Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Wearing pants is not permissible for a woman except if it meets the following conditions:
First, the pants should be covered with another loose outer garment, which covers them.
Second, the pants of women should not be similar to those of men.
Third, the purpose of wearing pants should not be to imitate non-believing women.
Whenever the above conditions are met wearing pants becomes permissible.
Know that the dress of the woman should always be thick, loose and large enough to cover her whole body.
In addition to the above conditions if the dress is worn outside of her house it should not have any particular features that make it attractive or unusual.
All these conditions are set for the woman to protect her only and to protect her honour as well from humiliation or from being ravished by evil persons.
Finally a Muslimah should totally submit herself to the Commands of Allah. And should believe firmly that all benefits are only in following the Shariah, and all disadvantages are in disobeying Allah.
Please refer to the Fatwa: 81209.
Allah knows best.