What is the plural posessive of women?

The plural possessive of woman is women's.

Normally the plural possessive in English is formed by adding s' (s + apostrophe) to the noun, but plurals that do not end in s, like men, women, children and oxen, simply add 's(apostrophe + s), just like singular possessives. Thus you have dogs' collars and priests' vestments, but women's magazines and children's toys.