A baby’s sex is determined by which sperm fertilizes the egg. Every egg carries an X chromosome, while sperm carry either an X or a Y. When an X-bearing sperm reaches the egg first, the result is a girl (XX). When a Y-bearing sperm wins, you get a boy (XY). The body produces X and Y sperm in a roughly 50:50 ratio, so without intervention, the odds of having a girl are essentially a coin flip.
That hasn’t stopped people from trying to tip those odds. Several popular methods claim to increase your chances, ranging from timing intercourse to choosing specific sexual positions to undergoing fertility procedures. Here’s what science actually says about each one.
Why It’s Always the Sperm That Decides
During sperm production, cells divide in a way that creates equal numbers of X-bearing and Y-bearing sperm. The two types are nearly identical in structure, function, and behavior. The only measurable difference is a small variation in DNA content, since the X chromosome is physically larger than the Y chromosome. Despite popular belief, studies have found no consistent shape differences between the two types.
This matters because many natural sex selection methods are built on the assumption that X and Y sperm behave differently, that one type swims faster, lives longer, or thrives in different conditions. The scientific evidence for those assumptions is thin.
The Shettles Method
The most widely known natural approach comes from Dr. Landrum Shettles, who proposed that Y-bearing sperm are faster but more fragile, while X-bearing sperm are slower but hardier. Based on this theory, couples trying for a girl should have intercourse two to four days before ovulation, then stop. The idea is that by the time the egg is released, only the longer-lasting X sperm would still be viable. Shettles also recommended shallower penetration during sex and avoiding female orgasm, both intended to create conditions less favorable to Y sperm.
Shettles claimed a 75 percent success rate for couples trying for girls. A 1979 study published in The New England Journal of Medicine involving over 3,000 births did find that boys were more often conceived when intercourse happened closest to ovulation, which would support the method’s logic. But a 1991 study in The American Journal of Obstetrics and Gynecology found the opposite result: fewer boys when conception occurred at ovulation. Then a 1995 New England Journal of Medicine study found no association between intercourse timing and fetal sex at all.
The foundational claim that X and Y sperm differ in shape or swimming ability has also been challenged. Research conducted after Shettles published his recommendations showed no detectable shape differences between sperm carrying X and Y chromosomes. Without that biological basis, the method rests on shaky ground.
The Whelan Method
Elizabeth Whelan proposed a competing timing theory that partly contradicts Shettles. Her method recommends having intercourse two or three days before ovulation to increase the chances of a girl. While this overlaps somewhat with Shettles’ advice, Whelan’s underlying reasoning differed, and neither method has been validated by consistent clinical evidence. The same 1995 study that undermined Shettles also applies here: no reliable link between intercourse timing and baby sex has held up across multiple trials.
Diet and pH Theories
Another popular idea is that a mother’s diet or vaginal pH can favor one type of sperm over the other. Animal studies have found some intriguing patterns. In rats, a maternal diet high in calcium and magnesium but low in sodium and potassium shifted the sex ratio. In hamsters, vaginal pH around the time of mating appeared to influence which sperm succeeded. The theory extended to humans suggests that an acidic vaginal environment favors X-bearing sperm (and therefore girls), while an alkaline environment favors Y-bearing sperm.
Some proponents recommend eating calcium-rich and magnesium-rich foods like dairy, leafy greens, and nuts while avoiding salty and potassium-heavy foods. Others suggest acidic douches before intercourse. The problem is that human reproductive tracts are far more complex than those of rodents, and no controlled human studies have demonstrated that dietary changes or pH manipulation reliably shift the odds of conceiving a specific sex. The cervical mucus, nutrient content of reproductive tract fluids, and precise timing of intercourse all interact in ways that haven’t been untangled in clinical research.
Sperm Sorting Technology
A more scientific approach involves physically separating X-bearing sperm from Y-bearing sperm before insemination. The most well-known version, called MicroSort, uses a laser-based sorting technique that detects the slight DNA difference between X and Y sperm. In clinical trials involving over 1,000 babies born, 93.5 percent were female when sperm had been sorted for X chromosomes. Sorted samples averaged about 88 percent X-bearing sperm.
However, MicroSort’s path has been complicated. The FDA acknowledged the technique was safe and effective but blocked its commercial use in the United States in 2011, citing no “public health benefit” to offering sex selection for nonmedical purposes. The technology has been available in some other countries, but access varies and typically requires pairing with intrauterine insemination or IVF. Pregnancy rates per cycle were modest: around 15 percent with insemination and 31 percent with IVF.
IVF With Genetic Testing
The most reliable method for choosing a baby’s sex is IVF combined with preimplantation genetic testing. During this process, eggs are fertilized in a lab, embryos develop for several days, and then a few cells are biopsied from each embryo. The testing reveals the chromosomal makeup, including whether the embryo is XX or XY. Only embryos of the desired sex are then transferred to the uterus.
This approach is nearly 100 percent accurate, though the American Society for Reproductive Medicine notes that rare diagnostic errors can occur. It is by far the most effective option, but it’s also the most invasive and expensive, typically costing tens of thousands of dollars per cycle. It involves hormone injections, egg retrieval under sedation, and the full IVF process regardless of whether you have any fertility issues.
Where Sex Selection Is Legal
The legal landscape varies dramatically. The United States has no laws expressly restricting sex selection for nonmedical reasons, and IVF with genetic testing for sex selection is widely available at fertility clinics. The FDA did not ban IVF or genetic testing the way it restricted MicroSort.
Most other countries take a stricter approach. As of the most recent comprehensive review, 31 countries prohibit nonmedical sex selection, including the United Kingdom, Canada, Australia, France, Germany, India, and China. In the UK, the Human Fertilisation and Embryology Act requires that clinics only perform sex selection for medical reasons, such as avoiding sex-linked genetic diseases. Five countries, including Switzerland and New Zealand, ban sex selection under any circumstances.
This legal gap has created a phenomenon sometimes called “reproductive tourism,” where people travel to the United States or other permissive jurisdictions specifically for sex selection procedures.
What the Evidence Actually Supports
If you’re hoping for a girl, the honest picture is this: no natural method has been proven to reliably shift the odds beyond the baseline 50 percent. The Shettles method is the most popular, but the studies examining it contradict each other, and its biological assumptions about sperm differences haven’t held up. Diet and pH theories are based on animal research that hasn’t translated to humans in any controlled way.
The only methods with demonstrated effectiveness are medical ones. Sperm sorting produces girls roughly 93 percent of the time but has limited availability. IVF with genetic testing is nearly certain but comes with significant cost, physical demands, and in most countries outside the United States, legal restrictions. For couples without access to or interest in these procedures, natural methods are unlikely to do harm, but the evidence suggests they’re unlikely to do much of anything else either.