Scientists have used the size difference between the X and Y chromosomes to develop technologies that may help people choose the sex of their babies. Some techniques use dyes to differentiate sperm carrying the X from those carrying the Y. Others use swimming speeds or centrifuges to sort the slower, heavier X-containing sperm from the faster, lighter sperm carrying the Y chromosome.
After Y-carrying sperm are separated from X-carrying sperm, the woman is inseminated with the desired fraction of the sperm. Alternatively, if a woman is using in vitro fertilization, the egg is fertilized in the laboratory with the sperm most likely to give a baby of the desired sex. None of the methods, however, works all the time; at best, the sperm samples are enriched in sperm that favor one sex over the other.
What about “natural” ways to boost the odds of having a boy or a girl? Folk wisdom includes advice on the mother’s diet (including lists of “boy foods” and “girl foods”), the timing of intercourse, the sexual position, and even what objects to place under the pillow during intercourse. Some of these ideas may have a biological basis, and it may be possible to give a slight edge to X- or Y-containing sperm in the race to fertilize the egg.
But the evidence for all of these “do-it-yourself” techniques relies heavily on anecdotes. Testimonials like “It worked for me!” may seem compelling, but the odds of having a baby with the desired sex are near 50% no matter what a person does. In reality, all such methods are difficult or impossible to test using controlled experiments,
so it is hard to know for sure what (if anything) works.