Praise be to Allaah.  

Muslim woman have to wear clothes that will conceal the body and her ‘awrah, by wearing clothes that do not show the skin, as see-though clothes do, and do not show the shape of the body, as tight clothes do. Pants or trousers are clothes that show the shape of a woman’s body and ‘awrah, so for this reason it is not permissible for women to wear pants, unless she wears over them a wide or loose shirt. One of the aims of Islam is protect people’s ‘awrahs and avoid uncovering them, because taking this matter lightly is one of the means of falling into that which Allaah has forbidden, namely zina (fornication, adultery) and the things that lead to it. The Muslim woman has to adhere to the Islamic etiquette in the way she dresses, moves and speaks. Allaah says (interpretation of the meaning): 

“O Prophet! Tell your wives and your daughters and the women of the believers to draw their cloaks (veils) all over their bodies”

[al-Ahzaab 33:59] 

“And tell the believing women to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts) and not to show off their adornment except only that which is apparent (like both eyes for necessity to see the way, or outer palms of hands or one eye or dress like veil, gloves, headcover, apron), and to draw their veils all over Juyoobihinna (i.e. their bodies, faces, necks and bosoms) and not to reveal their adornment ... And let them not stamp their feet so as to reveal what they hide of their adornment. And all of you beg Allaah to forgive you all, O believers, that you may be successful”[al-Noor 24:31] 

And Allah knows best.