The Shettles method is a natural sex selection technique based on timing intercourse relative to ovulation to influence whether you conceive a boy or a girl. Developed by Dr. Landrum Shettles, the method rests on the idea that sperm carrying the Y chromosome (boy) and sperm carrying the X chromosome (girl) have different physical characteristics that make them better suited to different conditions. Dr. Shettles claimed an 80% success rate among his patients, but independent research has not supported those numbers.
The Theory Behind the Method
Shettles proposed that Y-bearing sperm (which produce boys) have smaller heads and swim faster, while X-bearing sperm (which produce girls) are larger, slower, but longer-lived. From these supposed differences, he reasoned that Y sperm would reach the egg first if intercourse happened close to ovulation, while X sperm would outlast Y sperm if intercourse happened a few days earlier, when the egg hadn’t yet been released.
He first published these observations in the 1960s after examining sperm under a microscope and concluding that smaller-headed sperm carried the Y chromosome while larger-headed sperm carried the X. His book, “How to Choose the Sex of Your Baby,” popularized the method and made it one of the most widely known natural sex selection approaches.
How to Try for a Boy
To conceive a boy using the Shettles method, you would avoid intercourse from the start of your menstrual cycle until the day of ovulation. Then have sex on the day you ovulate and up to two to three days after. The logic is that the faster Y sperm will reach the egg first when the distance and timing are short.
This means you need a reliable way to pinpoint ovulation. Common approaches include tracking basal body temperature daily (your temperature rises slightly after ovulation), monitoring cervical mucus (it becomes clear and stretchy around ovulation), or using over-the-counter ovulation predictor kits that detect hormone surges in urine.
How to Try for a Girl
For a girl, the method works in the opposite direction. You would have sex in the days after menstruation ends but stop at least two to three days before ovulation. Then abstain during and after ovulation. The idea is that by the time the egg is released, the faster Y sperm will have already died off, leaving the hardier X sperm to fertilize it.
This approach requires even more precise cycle tracking, since you need to predict ovulation days in advance rather than simply detecting it when it happens. For people with irregular cycles, this can be particularly difficult to execute consistently.
What Science Says About Sperm Differences
The biological foundation of the Shettles method has been a source of debate for decades. Shettles originally claimed that sperm fell neatly into two size categories, with all X sperm being visibly larger than all Y sperm. Later researchers were unable to replicate this clean separation.
More sophisticated studies using methods to identify individual sperm as X or Y-bearing have found that X sperm heads are, on average, about 6% larger in area than Y sperm heads, and X sperm have slightly longer tails and necks. Human X sperm also contain roughly 3 to 4% more DNA than Y sperm. So a real size difference does exist, but it’s a statistical average with significant overlap between individuals. You can’t look at a single sperm cell and reliably determine whether it’s carrying an X or Y chromosome based on size alone. A 2006 analysis published in The BMJ described Shettles’ original claims as “entrenched misinformation,” noting that subsequent investigators could not confirm his findings about distinct size groups.
More importantly, the assumption that smaller sperm swim faster has never been convincingly demonstrated in humans. The size differences are so small that they don’t translate into meaningful speed advantages under real-world conditions inside the reproductive tract.
Does the Method Actually Work?
Dr. Shettles reported that 80% of couples using his method conceived a baby of their desired sex. However, he acknowledged it was not a guarantee, and these results came from his own patient observations rather than controlled studies.
Independent research tells a different story. A 2016 study found no evidence that the pattern or timing of intercourse has any effect on the sex of the resulting baby. The American Society for Reproductive Medicine’s committee opinion is similarly direct: “There is no convincing evidence for any relationship between specific coital practices and infant gender.”
Since the baseline odds of having a boy or girl are already close to 50/50, any method will appear to “work” about half the time. Without rigorous controlled trials comparing outcomes to chance, an 80% success rate is difficult to verify. Couples who try the method and get the result they wanted are naturally more likely to share their story, which creates a perception of effectiveness that isn’t supported by the data.
Practical Considerations
Even setting aside the question of whether the method influences sex, following the Shettles approach changes your conception window in ways worth understanding. If you’re trying for a girl, you’re deliberately avoiding sex during your most fertile days, which could reduce your overall chances of getting pregnant in a given cycle. The two to three days before and the day of ovulation are when conception is most likely, and the method asks you to skip that window entirely.
If you’re trying for a boy, the method concentrates intercourse around ovulation day, which aligns well with peak fertility. So couples following the “boy” protocol may not see any reduction in their chances of conceiving, while those following the “girl” protocol might find it takes longer to get pregnant.
The method is free, noninvasive, and carries no health risks, which is part of its enduring appeal. For couples who want to feel like they’re doing something proactive, it offers a sense of agency. But it’s worth going in with realistic expectations: the scientific consensus is that timing intercourse does not meaningfully shift the odds from the natural near-50/50 split.