We do know that the biological gender of a baby is determined long before that. In fact, it's determined upon the fertilization of a mother's egg. The 23rd pair of chromosomes makes that determination. An XY combination means that a child will be male, while an XX combination means that a child will be female.
Gender-related issues are not always so black and white. After an egg is fertilized and physical organs develop, there are still complicated questions raised about a person's identity.
There is quite a lot of debate about the concept of gender, including the way that word itself should be defined. Some support the idea that gender identity is environmental, meaning that it is determined by common behavior or activity that a child observes and learns, such as playing with dolls vs. trucks. Others argue that identifying with one gender is caused mainly by biological factors.
Whether one of these theories is right and the other is wrong, or if the truth lies somewhere in between, has not been settled. It certainly has provided speculation and interesting research, though.
