What is the ruling on growing nails for women and men, and what is the wisdom behind its prohibition if it is indeed prohibited?

Trimming nails is one of the practices of Fitra (natural disposition), as narrated from the Prophet (peace be upon him), and it is among the ten practices of Fitra. The Prophet (peace be upon him) stipulated that nails should not be left untrimmed for more than forty days. The wisdom behind this is cleanliness and avoiding resemblance to disbelievers and animals. Lengthening nails and painting them with nail polish, or keeping one nail long, is a clear violation of the Fitra practice.