I was recently asked about the legitimacy of sperm sorting as a way to choose the gender of your child. This is a topic I had been researching for another project anyway, so I thought I would report my findings. This is a good topic for skeptics because it may seem dubious at first glance, but in fact there is serious science behind sperm sorting. But there is also some pseudoscience mixed in.

Genetic gender is entirely determined by the sperm from the male. Women have two X chromosomes and so can only contribute an X. Men have one X and one Y, and so can contribute one or the other, which determines the sex chromosomes of the child. (I wrote “genetic gender” because there are non-genetic hormonal factors in the womb and hormonal abnormalities that can influence sexual development.) Therefore roughly half of the sperm rushing to be the victor in the conception race carry one X chromosome and will result in a girl, while the other half carry one Y chromosome and will result in a boy.

If we could separate out the Ys from the Xs, then we could control the resulting gender of the child. The X chromosome is larger than the Y. Since each sperm is essentially just a tight packet of genetic material connected to a tail, the weight of each sperm is largely determined by the weight of each chromosome.

One method which is about 20 years old is called the Beltsfield Sperm Sexing Technology. In this technique the sperm are labeled with a fluorescent die that binds to DNA – so the X sperm will glow brighter than the Y. As the sperm flow through a column they are given either a positive or negative charge depending upon how brightly they glow. Then flow cytometry is used  – the sperm migrate along a column in a magnetic field so that the negatively charged sperm will go down one path and the positive down another. At the end you have two collections of sperm – one mostly X and the other mostly Y. This technique results in one collection of 90% X and another of 75% Y – so it is more successful in selecting a girl than a boy, but still better than 50%. The technique is used routinely in animal husbandry for producing cows or swine of the desired sex.

Another method for sorting sperm is through the use of a centrifuge – X sperm are heavier than Y so spinning in a centrifuge should sort them into different layers by weight. This makes perfect sense in theory, and there are many companies offering this service to couples, usually for 2-3 thousand dollars. However, there is no evidence that this method works. Evaluating the sperm after spinning shows that there is still a 50/50 ratio of X and Y sperm.

This technique is still being researched, however, more as a method of separating viable sperm from dead or immotile sperm. But by itself is only weakly effective. Other techniques are also being developed that have more promise. For example have the sperm flow through a device with a second stream of saline. Live motile sperm can cross to the other stream, while dead or immotile sperm cannot.

Interestingly, much of the literature on sperm sorting deals with animals. There is a huge commercial incentive for companies to be able to produce cows, for example, of the desired sex. In the industry flow cytometry is the method of choice, and spinning in a centrifuge is not relied upon. So that should be a good indication of the state of the science. But more importantly there is simply no published evidence to support the spinning method for sex selection, although it may play a role in enriching healthy sperm.

If you think the entire topic is  a bit frivolous, and couples should just play the gender lottery and take what they get, there are serious medical indications for sex sorting. For example, there are X-linked genetic diseases. A couple may wish to have a child but not risk a child with a horrible X-linked disease.

Sex sorting is a complex combination of legitimate science and gray-zone pseudoscience – not easy for a couple to sort out by themselves with Google alone as their ally. And there are companies willing to sort them from their money with unproven techniques.