The sex of a baby is determined by which sperm fertilizes the egg: sperm carrying an X chromosome produce a girl, while sperm carrying a Y chromosome produce a boy. At conception, the odds are essentially 50/50. Several natural timing methods claim to shift those odds in favor of a girl, and one medical procedure can guarantee it, though at significant cost. Here’s what the science actually supports.
What Determines a Baby’s Sex
Every egg carries an X chromosome. Sperm carry either an X or a Y. When an X-bearing sperm fertilizes the egg, the result is XX, a girl. When a Y-bearing sperm wins the race, the result is XY, a boy.
Research published in the Proceedings of the National Academy of Sciences estimates the sex ratio at conception is about 0.5, meaning roughly equal numbers of male and female conceptions. The slight male bias seen at birth (about 105 boys for every 100 girls) appears to result from higher female mortality during pregnancy, not from a surplus of Y-bearing sperm reaching the egg. So you’re starting from a true coin flip.
For years, popular methods have tried to exploit supposed differences between X and Y sperm. A 2019 study did find that nearly 500 genes are active only in X-bearing sperm, and researchers identified receptors that could chemically slow one type of sperm without harming the other. But this is a laboratory technique, not something you can replicate at home. The key biological difference is genetic, not something as simple as “girl sperm swim slower” or “boy sperm die faster.”
The Shettles Method
The most widely known natural approach is the Shettles method, developed by Dr. Landrum Shettles starting in the 1960s. His theory rested on the idea that Y-bearing sperm are smaller and faster but die sooner, while X-bearing sperm are larger, slower, and longer-lived. To conceive a girl, Shettles recommended having intercourse two to four days before ovulation, then abstaining as ovulation approaches. The logic: by the time the egg is released, only the hardier X-bearing sperm would still be alive in the reproductive tract.
Shettles also suggested shallower penetration during intercourse (to deposit sperm farther from the cervix) and avoiding female orgasm, which he believed created conditions favoring Y sperm. The sixth edition of his book, “Your Baby’s Sex: Now You Can Choose,” claimed a 75 percent success rate for couples trying for a girl.
There’s a significant problem with this, though. A study conducted after that edition was published found no shape or size differences between X-bearing and Y-bearing sperm. The foundational assumption of the method, that the two types of sperm behave differently in the body, hasn’t held up under modern microscopy. The 75 percent figure also came from self-reported data rather than a controlled clinical trial, which makes it hard to distinguish from chance combined with selective reporting.
The Whelan Method
Elizabeth Whelan proposed a competing timing method in 1977 that partly contradicts Shettles. Whelan recommended having intercourse two to three days before ovulation to increase the chance of a girl, which overlaps with some Shettles advice, but her reasoning was based on different biochemical research about how cervical conditions change throughout the cycle.
Whelan reported a 66 percent success rate for conceiving a girl. That’s better than 50/50 but notably lower than her claimed 86 percent rate for boys, and neither figure has been replicated in independent studies. Like Shettles, the Whelan method lacks strong clinical evidence.
Diet, pH, and Other Popular Claims
You’ll find advice online about eating certain foods (more calcium and magnesium for a girl, less sodium and potassium), changing vaginal pH with douches, or timing intercourse to specific days based on Chinese calendar charts. None of these approaches have reliable scientific support. Small studies occasionally find weak correlations between maternal diet and offspring sex, but the effects are too small and inconsistent to serve as a practical method. Douching can also disrupt the vaginal microbiome and increase infection risk, so it’s generally not recommended regardless of your goal.
Medical Sex Selection With IVF
The only method with near-certain accuracy is preimplantation genetic testing during in vitro fertilization. In this process, embryos are created outside the body, and cells from each embryo are tested to determine chromosomal makeup, including sex. Only embryos of the desired sex are transferred to the uterus.
This approach is effectively 99 percent accurate for sex selection. It was originally developed to screen for sex-linked genetic diseases (like hemophilia or Duchenne muscular dystrophy, which predominantly affect boys), but some clinics offer it for what’s called “family balancing,” where parents who already have children of one sex want to try for the other.
A related technology, sperm sorting through flow cytometry, can separate X-bearing from Y-bearing sperm before fertilization. When sorting for girls, this process achieves roughly 89 percent purity, meaning about 89 percent of the sorted sample carries X chromosomes. The sorted sperm is then used for insemination or IVF. This technology was developed under the brand name MicroSort and has been available in some countries, though availability varies.
The Ethics Conversation
The American Society for Reproductive Medicine’s current ethics guidance states that sex selection “should not be encouraged for nonmedical indications,” but also says practitioners are under no ethical obligation to either provide or refuse the service. In practice, this means clinic policies vary. Some fertility clinics will accommodate elective sex selection during IVF; others will only offer genetic testing when there’s a medical reason.
If you’re considering IVF specifically for sex selection, expect to discuss your reasons during the consent process. Clinics that do offer it are expected to apply their policies equally to all patients regardless of background. The cost of a full IVF cycle with genetic testing typically runs $15,000 to $25,000 or more, making it a significant financial commitment for a non-medical preference.
What This Means in Practice
If you’re trying to conceive a girl without medical intervention, the honest answer is that no natural method reliably shifts your odds much beyond 50/50. Timing intercourse a few days before ovulation (as both Shettles and Whelan suggest) is the most commonly recommended approach, and it’s harmless to try, but you should go in understanding that the science behind it is weak. The core claim that X and Y sperm behave differently enough in the body to be influenced by timing hasn’t been confirmed by modern research.
If having a girl is important enough to pursue with high confidence, IVF with genetic testing is the only proven path. It’s expensive and physically demanding, involving hormone injections, egg retrieval, and embryo transfer, but it works. For most couples, the practical approach is to try the timing methods with realistic expectations and be genuinely happy with either outcome.