Sperm cells determine the gender of the resulting babies. Fertilization of an egg with an X-chromosome bearing sperm will result in a girl while a Y-chromosome bearing sperm produces a boy.

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Y-bearing sperm cells are less dense and swims faster than X-bearing ones and those are the basis used in processing semen samples for sperm sex selection. Sperm sex selection can be performed for patients undergoing IUI or IVF in order to increase the odds of achieving the desired gender of their child.