the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
There is a hadîth related in Sunan al-Tirmidhî and Sunan al-Nasâ’î from `Alî prohibiting women from shaving their heads. However, it is a weak hadîth. In fact, al-Tirmidhî himself said that this hadîth is defective. There is another hadîth related by al-Khalâl that the Prophet (peace be upon him) forbade women from shaving their heads except in cases where they are suffering from some ailment of the scalp. This hadîth is also not authentic.
Nevertheless, most scholars hold the opinion that it is disliked for a woman to shave her head unnecessarily. Indeed, in most societies, shaving the head is unknown for women and only known for men. In such circumstances, it will be unlawful for her to shave her head, since it is unlawful in Islam for women to dress in a manner that according to the customs of her society is deemed to resemble the dress of men.
And Allah knows best.
Source: Islam Today