MythBusters: Are all embryos originally females? No!
This is a story that has been told for at least 40 years. It is nonsense! The rumor developed because when the fetus is viewed through ultrasound during its embryonic development, the distinguishing characteristics of sex are not yet present, but some see it as it would resemble female.
Before fertilization, a woman's egg is just an egg. It has no sex. Sexual identity is determined at fertilization when the genetic sex of the zygote (a fertilized egg) has been initialized by a sperm cell containing either an X or Y chromosome. Sperm contain either an XY set of sexual chromosomes, or an XX set (XX is female and XY is male). Until 7 weeks, human embryos have no sex. The embryo starts developing into a fetus from the eighth week onwards. All fetuses start out with either an XY or XX set of chromosomes. It is the presence of chemicals (due to Y chromosome) that forms the male organs later on. When the Y chromosome is stimulated, testicles will begin to develop, elongation of penile tissue commences, and the interior aspects are stimulated to grow. If there is no Y chromosome present, then ovarian tissue begins to grow.