Gender is a social construct, not a biological fact, like sex. 'Gender' is defined by Merriam-Webster's as "the behavioral, cultural, or psychological traits typically associated with one sex." Gender identity is formed as a result of the roles we believe we ought to play based on the social expectations associated with our sex.
Gender identity is expressed in multiple ways -- in how we dress and groom ourselves, for example. Gender roles generally tell us that men ought to be masculine, and women ought to be feminine. Starting in the twentieth-century, particularly after the second-wave feminist movement, we have begun to question what it means to be masculine and feminine. We are also seeing the ways in which both sexes can have masculine and feminine characteristics.
Gender similarities and differences are individually determined. This means that not all women (this includes cisgender and trans-women) present femininity in the same way, and not all men (cisgender and trans) present masculinity in the same way.
One gender role that is commonly associated with women is the role of mother. There is still a social expectation that women of a certain age are supposed to have children. Women who have children still, according to data, take on more responsibility for child-rearing and household chores than men do. In the United States, data has also shown that women who have children are less likely to receive raises and promotions at work, while men who have children do receive these benefits, due to the perpetual notion of men as providers.