Sperm spinning is a process which is designed to separate male and female sperm, allowing parents who are using the assistance of technology to reproduce to select the gender of their baby. Like many techniques used to assist couples who have trouble getting and staying pregnant, sperm spinning is not guaranteed to be effective, but it can greatly help to increase the chances of having a baby of the desired gender. Because gender selection is a fraught issue in many societies, sperm spinning is not without controversy, and many parents who choose to use sperm spinning do not discuss it with people outside the family.

The idea behind sperm spinning is fairly simple. Sperm with an X chromosome tends to be heavier than sperm with a Y chromosome, so, in theory, spinning sperm in a centrifuge will separate out the male and female sperm, with the female sperm sinking to the bottom of the centrifuge. Once the sperm spinning process is complete, the sperm of choice can be introduced to an egg to create an embryo which may be implanted in the mother or a surrogate, depending on the family's situation.

In practice, sperm spinning is a bit more complicated, and several processing techniques are available for couples who want to pursue this treatment. The effectiveness of sperm spinning is also a topic of debate, with some studies suggesting that it is not terribly effective, with the chance of having a girl or a boy after sperm spinning being about equal. However, some practitioners have shown that they can use the technique reasonably effectively, ensuring that it remains popular in the fertility treatment community.

This technique was originally designed for parents who were concerned about the possibility of passing on dangerous sex-linked genetic traits. For example, in a family with a history of hemophilia, having a boy is a cause for concern, as there is a chance he will be a hemophiliac, whereas girls can only carry the disease. Gender selection can be used to avoid several other sex-linked diseases as well, reducing strain on parents who may be concerned about having unhealthy children.

However, sperm spinning is also, of course, used to select children of a desired gender. In some cases, parents request a specific gender to balance an existing family, while in other instances, they simply have a desire for one gender over the other. This practice is frowned upon by some people, who believe that children of all genders should be welcome, and the issue of gender selection raises some uncomfortable issues for some communities, especially in Asia, where boys have traditionally been highly valued over girls.