This is going to come as a shock to followers of the Shettles gender selection method: research using the most reliable and advanced techniques for analyzing human sperm has revealed that there is no difference in the size and shape of X and Y sperm -- a finding in direct conflict with Shettles' main principle.
The study appears in the latest issue of Fertility and Sterility, the respected journal of the American Society for Reproductive Medicine (ASRM). The conclusion agrees with some previous studies showing that X and Y sperm cells have the same morphology (size, shape, and structure).
So what does this mean for the Shettles method? In his book, Dr. Shettles claims the size difference between the two types of sperm -- X and Y, with the X's responsible for the conception of a girl and the Y's causing the conception of a boy -- is the key to swaying the odds in favor of conceiving the gender of your choice. Because the Y chromosome is smaller than the X chromosome, Shettles theorized, the Y-bearing sperm should be smaller and faster. The greater mass of the X sperm, on the other hand, should make it slower but hardier and better able to overcome obstacles to fertilization.
There's no dispute that the X chromosome is bigger than the Y chromosome, as this photo clearly shows. But the sex chromosome is only one of the 23 chromosomes carried by a sperm -- in total, an X sperm carries only 2.9% more genetic material than a Y sperm. As you can see, the difference isn't that great.
Total DNA in X Sperm
Total DNA in Y Sperm (2.9% less)
Data from the study shows that, not only are X and Y sperm indistinguishable from one another, even abnormal sperm cells with the wrong number of chromosomes -- too many or too few chromosomes -- cannot be identified by having a different shape or size.
Shettles describes how he observed two distinct populations of sperm: smaller, pointier sperm that he assumed were Y sperm, and bulkier, rounder sperm that he assumed were X sperm. However, Shettles never proved that the different-appearing sperm cells he saw actually corresponded to X and Y sperm. We now know for a fact that it is impossible to identify a living X from a Y sperm just by looking at it with a microscope, no matter how powerful the microscope is. (If it were, we'd have a reliable method of gender selection without resorting to PGD.)
The truth is that at the time Dr. Shettles conducted his interesting sperm experiments -- more than 4 decades ago -- there didn't exist an accurate method of detecting X and Y sperm in a semen sample.
Two sperm at different stages of maturity
What has been shown by recent research is that sperm shape indicates maturity, not whether the sperm is carrying an X or Y chromosome. Sperm cells which have undergone "capacitation", which occurs after ejaculation and enables a sperm cell to fertilize an egg, are pointy. Sperm cells which have not undergone this process are round and enlarged, and cannot fertilize an egg.
It seems clear that Dr. Shettles sincerely wanted to find a method that would help parents choose the sex of their baby. But was his theory founded on wishful thinking? Did his dream of finding a useful difference between X and Y sperm lead him to make a false assumption when he saw differently-shaped sperm cells under the microscope?