The earliest an ultrasound can possibly determine the gender of a fetus is 12 weeks. Most doctors won't attempt to determine the gender until weeks. Unlike amniocentesis, ultrasound genderizing isn't an exact science. Depending on the way the baby lays, it can be difficult to see the genitals. Some radiologists will influence the baby to move into a different position to catch a glimpse, or you could consume something containing sugar to make the fetus more active. Amniocentesis, if you have to have one done, determines the gender through chromosomal analysis and this is fool-proof.
Gender and Genetics
How do you determine the sex of a fetus - Answers
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Sexual differentiation in humans
Sexual differentiation in humans is the process of development of sex differences in humans. It is defined as the development of phenotypic structures consequent to the action of hormones produced following gonadal determination. The development of sexual differences begins with the XY sex-determination system that is present in humans, and complex mechanisms are responsible for the development of the phenotypic differences between male and female humans from an undifferentiated zygote. At an early stage in embryonic development, both sexes possess equivalent internal structures. These are the mesonephric ducts and paramesonephric ducts.