It is impossible to tell the different between male and female embryos until they are between eight and ten weeks old. There are no outwardly visible signs until after this time, so an ultra sound would not be able to tell.

What happens after twelve weeks will depend on the type of chromosomes which have been inherited from the parents of the embryo.  A pattern of two X chromosomes, known as XX, means that the inherited chromosomes are female.  If they are made up of one Y and one X chromosome, the scientific pattern is XY and the embryo will be male.

The embryo always inherits an X chromosome from the mother, but the father can pass on another X or a Y chromosome. So it is the father who determines the sex of the baby.  This natural selection of chromosomes appears to be random but scientists are still examining the evidence to find out exactly what determines the type of chromosome that the father passes on.  Families where there are all girls, or all boys, may eventually give the clue to how selection is made.

After twelve weeks, depending on the chromosome pattern, testes or ovaries are produced. If testes develop they make a chemical which stops the female reproductive organs from developing.  Testosterone is also produced and this stimulates the growth of male reproductive organs.

Chromosomes do not simply determine the sex of a baby. They also determine physical strength and character and if the balance between the chromosomes is abnormal the baby develops mixed characteristics. For example, tests done on some female athletes in the past showed abnormal XXY chromosome patterns.  This in effect means that they have a third male chromosome as well as their two female ones.   This gives them extra strength and men with an extra Y chromosome giving them an XYY pattern are usually bigger and more aggressive than average.  In fact there has been research in the US which suggests that men with this extra Y chromosome may be more likely to be involved in crime.

So, it seems that there may actually be four sexes instead of just the two we have been accustomed to collecting. There is XX, XY, but then there is XXY and XYY. This certainly gives a great deal of food for thought.