The Living Force
If all fetuses start out as female, and certain hormonal stimulation creates "gender", then it is the amount of stimulation that determines whether the fetus becomes "male". My obeservation is that we are already constituted of both male and female contributions from our parents. It is the amount of hormonal input that determines how much femininity or masculinity one has. I always wonder about "immaculate" conception and in most cases I have read, the child is female as there was no y factor contribution. When men have babies where does the child exit? There is no natural way for a man to conceive and birth a child.
Quote from Livestrong
Gender is determined immediately upon fertilization. The 23rd pair of chromosomes establishes the sex of the baby. The mother's egg contains an X chromosome, while the father's sperm carries either another X or a Y chromosome. An XX combination means your baby is female and an XY combination means your baby is male. This means that the baby's gender is determined before it is even considered a fetus.