May women wear pants?
It is forbidden for women to wear pants. This applies even towards women pants that are made specifically for women and are not worn by men. In the latter case it is disputed as to whether this prohibition is due to Lo Silbash or only due to reasons of modesty.
Under a skirt: Women are not to initially wear pants outside even for protection from the cold and even if they wear a skirt on top. However, in a time of need, one may be lenient. [See below regarding skiing]
Children: It is forbidden for girls who have reached the age of three years old to wear pants. Some Poskim rule it is even forbidden prior to this age. Practically, girls below three years old may wear girl pants or unisex pants, however they are not to wear pants of boys unless there is no other clothing available and they need to wear it to protect from the cold or the sun.
May a woman wear pants while skiing?
Some Poskim rule it is forbidden to do so. Practically, however, if she is wearing it in order to protect her from the cold or for Tzenius purposes, then she may wear pants under a skirt for extra modesty and protection. It is forbidden to go skiing without a skirt due to reasons of modesty.]
May women wear pajama pants?
Some Poskim rule it is forbidden for women or girls to wear pajama pants even at night in an only girl’s room, and even if it is worn under a nightgown, unless she is wearing it to protect from the cold. Other Poskim however rule that pajama pants are permitted to be worn in private, while not in the presence of other men. Practically, it is permitted for women and girls to wear women pajama pants in private, under a nightgown or long T-shirt, for Tzenius purposes or in order to protect from the cold, and so is the widespread custom even amongst Chassidic and G-d fearing women. They are however not to wear the pants in front of other men that are in the house.
 Minchas Yitzchak 2/108; Shevet Halevi 2/63; See Ohel Yaakov 188/44-45
 The reason: As pants is mainly a men’s clothing and hence it is considered Beged Ish for a woman to wear it even if it is a woman’s style. [Minchas Yitzchak ibid; Shevet Halevi ibid] In addition, these clothing are Bigdei Zima, clothing of promiscuity which shape a woman’s legs, thighs and private area. [Minchas Yitzchak ibid; Shevet Halevi ibid] Some Poskim however rule that women pants which are different than men pants do not contain Beged Ish as it is not similar to men’s clothing, although it would nevertheless be forbidden due to being immodest. [See Avnei Tzedek Y.D. 72; Mishneh Halachos 12/353]
 See previous footnote
 The reason: Although doing so is not forbidden due to Beged Ish, being she is wearing it for protection from the cold, nevertheless it is not a modest and elegant form of dress.
 Avnei Tzedek Y.D. 73; Shevet Halevi 2/63; See Minchas Yitzchak ibid
The reason: As according to most Poskim it does not transgress Lo Silbash being she is not intending to wear it in order to resemble a man, and even according to the dissenting Poskim, if she is wearing women pants, perhaps they do not at all contain a prohibition of Lo Silbash. Regarding modesty, since the pants is covered by a skirt it does not present any prohibition.
 The reason: As starting from three years of age we educate a girl to dress modestly. Furthermore, once she reaches the age of Chinuch, it is forbidden for her to wear men’s clothing.
 Minchas Yitzchak 2/108
 The reason: As it is forbidden to feed children a prohibition even from one day old. [See Admur 343/2] Furthermore, by Lo Silbash perhaps the person who places the clothing onto another also transgresses. [Minchas Yitzchak ibid]
 Minchas Peri 2/66; Poskim brought in Ohel Yaakov 292 footnote 36
As a) They are different than boys pants and do not contain Lo Silbash [Avnei Tzedek Y.D. 72 and perhaps even according to Minchas Yitzchak 2/108 and Shevet Halevi 2/63 they would agree that by a toddler it is not considered at all a boys clothing any more than a girl being that so is the common practice] B) There is no immodesty involved as they are below the age of three.
 See Minchas Yitzchak ibid
The reason: As it is forbidden for an adult to dress even a child with men’s clothing as it is forbidden to feed a prohibition to a child of any age.
 Minchas Yitzchak 2/108
 The reason: As although many Poskim rule a woman may wear men’s clothing to protect her from the cold and the like, nevertheless here she is placing herself into the situation, and in this case according to all Poskim we tell her simply not to go skiing and thus not have a need to wear the pants, [Minchas Yitzchak ibid]
 See Avnei Tzedek Y.D. 73; Shevet Halevi 2/63
The reason: As according to most Poskim it does not transgress Lo Silbash being she is not intending to wear it in order to resemble a man. It is difficult to accept the argument of the Minchas Yitzchak ibid as if so “Nasnu Devarecha Leshiurim.”
 Minchas Yitzchak 2/108
 The reason: As the prohibition of Lo Silbash applies even when wearing the clothing in private, not in front of men. [ibid]
 See Avnei Tzedek Y.D. 73 and Rivivos Efraim 5/534 and Ohel Yaakov p. 308 in name of Rav Elyashiv and Rav Wozner and many Poskim of today that pajama pants do not contain the prohibition of Lo Silbash as they are not men’s clothing at all and are not made to appear in public
 Avnei Tzedek Y.D 73
The reason: As according to most Poskim it does not transgress Lo Silbash being she is not intending to wear it in order to resemble a man.