The Shettles method does not reliably influence the sex of a baby. Developed in the 1960s and popularized through a best-selling book, the technique is built on biological assumptions about sperm that modern research has repeatedly failed to confirm. Multiple studies, including a large analysis published in the New England Journal of Medicine, have found that the timing of intercourse relative to ovulation has no meaningful effect on whether a conception produces a boy or a girl. The method remains widely discussed online, but the evidence against it is extensive and consistent.
What the Shettles Method Claims
The method rests on the idea that sperm carrying the Y chromosome (which would produce a boy) are smaller, faster swimmers but die sooner, while sperm carrying the X chromosome (which would produce a girl) are larger, slower, but hardier. From these supposed differences, Landrum Shettles recommended that couples wanting a boy should have intercourse as close to ovulation as possible, so the speedy Y-bearing sperm reach the egg first. Couples wanting a girl, by contrast, should time intercourse two to three days before ovulation, giving the slower but longer-lived X-bearing sperm the advantage after the Y-bearing sperm have died off.
The method also incorporates advice about vaginal pH and sexual position. Shettles suggested that an alkaline environment (achieved through douching with baking soda, or through deeper penetration near the cervix) favors Y-bearing sperm, while a more acidic environment favors X-bearing sperm. Female orgasm was said to shift vaginal pH toward alkaline, theoretically boosting the chances of a boy. These recommendations, along with the timing guidance, formed a package that appealed to couples because it required no medical intervention and cost nothing to try.
The Core Biological Premise Is Wrong
The entire framework depends on Y-bearing sperm being smaller and faster than X-bearing sperm. That difference does not exist in any way researchers have been able to measure. A commentary in the BMJ put it bluntly: despite the “complete absence of scientific evidence for differential motility, or even for a difference in size,” the myth persists that X and Y sperm behave differently enough to exploit for sex selection.1PubMed Central. Entrenched misinformation about X and Y sperm The X chromosome is indeed physically larger than the Y chromosome, but the DNA mass difference between the two is a fraction of a percent of the total sperm cell mass. Under scanning electron microscopy, researchers have been unable to tell X-bearing and Y-bearing sperm apart.2PubMed. The effects of altering the pH of seminal fluid on the sex ratio of rabbit offspring
There is one laboratory finding that partially touches on differential viability. A 2017 study incubated human sperm for three days at elevated temperature and pH and found that Y-bearing sperm showed more signs of programmed cell death under those specific conditions.3Human Reproduction. Sex chromosome-dependent differential viability of human spermatozoa during prolonged incubation But three days at 37°C in a lab dish is a far cry from what sperm experience inside the reproductive tract, where conditions are dynamic and the journey to the egg takes hours, not days. The finding is interesting for reproductive biology but does not validate the Shettles framework for couples trying to conceive naturally.
Timing Intercourse Does Not Shift the Sex Ratio
If the Shettles method worked, you would expect to see a clear pattern in large datasets: couples who conceived closer to ovulation should have more boys, and couples who conceived further from ovulation should have more girls. Study after study has looked for that pattern and come up empty.
A widely cited study published in the New England Journal of Medicine tracked couples who were timing intercourse to conceive and found that cycles producing boys and cycles producing girls showed similar patterns of intercourse relative to ovulation. The researchers concluded that, for practical purposes, the timing of intercourse has no influence on the sex of the baby.4PubMed. Timing of Sexual Intercourse in Relation to Ovulation — Effects on the Probability of Conception, Survival of the Pregnancy, and Sex of the Baby A study of nearly 950 singleton births among couples using natural family planning found the same thing: the sex ratio did not vary consistently or significantly with the estimated timing of insemination relative to ovulation.5PubMed. Sex ratio associated with timing of insemination and length of the follicular phase in planned and unplanned pregnancies during use of natural family planning
One earlier study actually found a statistically significant trend in the opposite direction from what Shettles predicted: longer intervals between intercourse and fertilization were associated with more boys, not fewer.6PubMed. A prospective study of the preselection of the sex of offspring by timing intercourse relative to ovulation The authors stated explicitly that the results “clearly refute the theory that intercourse close to ovulation favors male conceptions.” Meanwhile, another analysis of natural family planning data found that among midcycle conceptions, about 86% were male, but the authors considered the effect too inconsistent and small to be useful for sex selection in practice.7PubMed. Natural family planning and sex selection: fact or fiction? The overall picture is one of no reliable or usable pattern.
The pH Argument Doesn’t Hold Up Either
The second pillar of the Shettles method, that vaginal or cervical pH can differentially favor one type of sperm, has also been tested and found wanting. A study that altered the pH of seminal fluid in rabbits found that sperm motility dropped as pH moved away from neutral in either direction, with acidic conditions being more damaging than alkaline ones. But the key finding was that pH did not selectively harm one sex’s sperm over the other. The researchers concluded it “seems unlikely that a direct effect of pH on sperm can be a single influence on the sex of offspring.”2PubMed. The effects of altering the pH of seminal fluid on the sex ratio of rabbit offspring
More recent work using bovine sperm and highly sensitive genetic testing (digital droplet PCR) confirmed the same conclusion. Sperm were allowed to swim up through media adjusted to a range of pH levels from 5.5 to 9.0, and the researchers measured the proportion of X-bearing and Y-bearing sperm in different fractions. The differences were tiny and not statistically significant, leading the authors to state that X and Y sperm “are not differentially influenced by just the pH in the female cervix at ovulation.”8PubMed Central. Effect of the pH in the enrichment of X or Y sex chromosome-bearing sperm in bovine The idea that a baking soda douche could tip the balance toward boys has no credible experimental support.
Why the Method Stays Popular Despite the Evidence
Roughly half of all babies are boys and half are girls, so any method of sex selection starts with a coin-flip baseline. If you try the Shettles method and get the sex you wanted, you had close to a 50% chance of that happening anyway. With no method at all, about 51% of live births are male and 49% female. That natural skew means couples “trying for a boy” with Shettles will succeed slightly more than half the time, and couples “trying for a girl” will succeed slightly less than half the time, entirely by chance.
Confirmation bias does the rest. Couples who get their desired outcome remember the method fondly and tell friends about it. Couples who don’t get their desired outcome tend not to broadcast the failure, or they rationalize that they must have mistimed things. Online forums are full of testimonials from people who credit Shettles for their result, but testimonials without a control group tell you nothing about whether the method actually moved the needle.
There is also a practical wrinkle that is easy to overlook: accurately pinpointing ovulation is harder than most people assume. A Cochrane review on timed intercourse found that while urinary hormone testing probably improves the odds of conceiving, fertility awareness-based methods like tracking basal body temperature or cervical mucus did not clearly increase live birth rates compared to untimed intercourse.9PubMed. Timed intercourse for couples trying to conceive If couples cannot reliably pinpoint ovulation even for the purpose of getting pregnant at all, the prospect of fine-tuning intercourse timing to a degree that would (theoretically) shift sex ratios is unrealistic.
What About Other “Natural” Sex Selection Approaches
The Shettles method is the most famous natural approach, but it is not the only one. Others include the Whelan method (which reverses Shettles’ timing advice, recommending intercourse further from ovulation for boys), the Billings ovulation method, and various pre-conception diets.10PubMed. Global sex selection techniques for family planning: a narrative review Commercial “gender selection kits” also exist, typically combining supplements, douches, and timing charts.
The dietary angle has a slightly more interesting evidence base than timing, though it remains far from settled. One small prospective study asked women trying for a girl to eat a diet low in sodium and high in calcium, combined with intercourse timed well before ovulation. Among women who adhered to the protocol strictly enough to meet a prediction rule, about 81% had girls.11PubMed. Female gender pre-selection by maternal diet in combination with timing of sexual intercourse–a prospective study That sounds impressive, but the study was small (32 qualifying women), not randomized, and combined diet with timing, making it impossible to isolate which factor (if any) was responsible. An animal study in rats found that a diet high in calcium and magnesium produced more female offspring compared to a diet high in sodium and potassium.12Pakistan Journal of Nutrition. Comparing the Effects of Sodium and Potassium Diet with Calcium and Magnesium Diet on Sex Ratio of Rats’ Offspring The mechanism, if real, remains unclear, and rat physiology does not always translate to humans. These findings are worth knowing about but are a long way from a reliable method.
Medical Technologies That Actually Work
If you want a high degree of certainty about a baby’s sex before conception or implantation, the technology exists, but it requires assisted reproduction. The most established approach is preimplantation genetic testing (PGT) combined with in vitro fertilization. Embryos are created through IVF, a few cells are biopsied, and genetic analysis reveals whether each embryo carries XX or XY chromosomes. Only embryos of the desired sex are transferred. Sex concordance rates in modern PGT protocols are extremely high, with one recent study reporting about 99% accuracy for sex determination among fresh embryos.13PubMed Central. Optimizing Non-Invasive PGT-A: A Multi-Factorial Approach for Enhanced Accuracy and Seamless Integration Into Clinical IVF
Sperm sorting is another option. The MicroSort technology, which uses flow cytometry to separate sperm based on DNA content (X-bearing sperm have about 2.8% more DNA), achieved roughly 88% X-bearing purity and 74% Y-bearing purity in sorted specimens. Among babies born after sorting and insemination, about 93% were female when sorting for girls, and about 85% were male when sorting for boys.14PubMed Central. The effectiveness of flow cytometric sorting of human sperm (MicroSort®) for influencing a child’s sex Those numbers are far better than anything a natural method can offer, though still not 100%, and the process involves clinical fertility procedures with their associated costs and demands.
A newer experimental approach uses multilayer density gradients to separate sperm, based on the idea that X-bearing sperm are slightly heavier. In one non-randomized clinical trial, ejaculates were processed through a four-layer gradient and then used for IVF with PGT-A to confirm the results.15PLOS ONE. A non-randomized clinical trial to determine the safety and efficacy of a novel sperm sex selection technique These methods are still being refined and are not widely available, but they represent the direction the field is moving, toward laboratory-verified techniques rather than timing-based guesswork.
The Ethical and Legal Landscape
Sex selection for non-medical reasons is legally prohibited in most countries. A survey of 36 countries with relevant laws or policies found that, with Israel as a qualified exception, none permitted non-medical sex selection. The Council of Europe’s Convention on Human Rights and Biomedicine explicitly states that assisted reproduction “shall not be allowed for the purpose of choosing a future child’s sex, except where serious hereditary sex-related disease is to be avoided.”16Human Reproduction. ESHRE Task Force on ethics and Law 20: sex selection for non-medical reasons In most jurisdictions, these prohibitions apply specifically to assisted reproductive technologies rather than to natural methods like diet or timing, which would be essentially unenforceable anyway.
The United States is an outlier. American reproductive medicine does not have a blanket legal prohibition on non-medical sex selection, and the American Society for Reproductive Medicine’s ethics committee has taken a more nuanced position: because the practice is “ethically controversial,” clinics are encouraged to develop their own policies, and practitioners are under no ethical obligation to either provide or refuse non-medically-indicated sex selection.17PubMed. Use of reproductive technology for sex selection for nonmedical reasons: an Ethics Committee opinion In practice, this means that couples in the U.S. can access PGT-based sex selection if they find a willing clinic, while couples in most of Europe, Asia, and elsewhere cannot. That regulatory gap is one reason the Shettles method and similar natural approaches maintain an audience worldwide: for many people, they feel like the only option, even if they do not work.
Does Anything About the Parents Naturally Influence a Baby’s Sex
Beyond deliberate selection attempts, researchers have explored whether parental characteristics naturally shift the sex ratio. The Trivers-Willard hypothesis, proposed in the 1970s, suggests that mothers in good condition should produce more sons (in species where males show greater reproductive variation), because a well-resourced son can outcompete others and produce many offspring, while a poorly-resourced son may produce none.18PubMed Central. The Trivers-Willard hypothesis: sex ratio or investment?
There is some suggestive data. An analysis of billionaire families found that they had about 60% sons, significantly more than the general population’s roughly 51%.19PLoS ONE. A Trivers-Willard Effect in Contemporary Humans: Male-Biased Sex Ratios among Billionaires But the effect is far from settled. A comprehensive analysis of Swedish data spanning 243 years, using multiple indicators of maternal stress, found no consistent relationship between stress and sex ratio at birth, and the authors described their results as “incompatible” with the Trivers-Willard hypothesis.20PubMed Central. Maternal stress and sex ratio at birth in Sweden over two and a half centuries Paternal age, another factor sometimes discussed in this context, has also shown no statistically significant association with sex ratio in population-based analysis.21Human Reproduction Open. Paternal age and neonatal outcomes: a population-based cohort study
The honest summary of this research is that even if some environmental or condition-based mechanism nudges the sex ratio slightly in large populations, the effect is so small that it is functionally useless for any individual couple trying to choose. The natural sex ratio is stubbornly close to 50/50 across nearly every population and condition researchers have studied. For couples who care deeply about having a child of a particular sex, the only approaches with demonstrated reliability involve a laboratory, a clinic, and a substantial investment of time and money. The Shettles method, whatever its intuitive appeal, does not belong on that list.