X chromosomes are the common link. All human ovum contain the X chromosome, and sperm are either an X or a Y. Gender is determined by the sperm.XX=Female, XY=Male
Actually, this sets the blueprint, not the outcome. All humans start off with the equipment for both sexes. With no hormonal interaction, the fetus becomes female. The Mullerian duct forms the uterus and the Wolffian duct shrivels. The introduction of two hormones are required to produce a male. One causes the the Wollfian ducts to form into the male genitalia and the other inhibits Mullerian to cause the Mullerian ducts to shrivel.
In experiments with rabbit fetuses, the testicles were removed at a certain point in development and the rabbits formed into fully functioning females that could reproduce, though they still had the XY chromosone.
So, XY doesn't tell the whole story. Becoming male requires more to happen during the gestation period and is therefore slightly less likely. So the odds would seem to favor female births.
But, maybe the odds are offset at the fertilization. The XYs are faster swimmers but die faster. The XX sperms are slower and live longer. Maybe the XYs make it to the egg slightly more often so that in the end the odds are about the same. More XY's to offset the possibility that hormones don't kick in to produce a male.