What the method claims
Dr Landrum Shettles proposed in the 1960s that sperm carrying a Y chromosome swim faster but die sooner, while X-carrying sperm are slower and hardier. Sex on ovulation day would therefore favour a boy, and sex two to four days earlier a girl. The calculator places those days on your cycle so you can see what the claim amounts to.
What the evidence says
When Wilcox and colleagues tracked 221 pregnancies with daily hormone measurements and intercourse records, the sex ratio was flat across the fertile window. Laboratory work has since found no meaningful difference in speed or survival between X and Y sperm. Reviews of Shettles-style timing report success rates at chance. The one real effect of following the girl advice is a lower chance of conceiving at all, because it skips the two best days.
If what you want is a baby, the best time to get pregnant calculator gives the days that matter. If what you want is a guess for fun, the Chinese gender predictor is at least honest about being a coin flip too.
Frequently asked questions
Does the Shettles method work?
No. The method claims that sex on ovulation day favours a boy because Y sperm are faster, and sex two to four days before favours a girl because X sperm survive longer. When researchers checked, in the 1995 Wilcox study of 221 pregnancies with known intercourse timing and in later work, the proportion of boys was the same on every day of the fertile window. It is a coin flip whatever you do.
Can you choose your baby's sex naturally?
Not with timing, diet, positions, or douching, despite what many sites claim. The only methods with evidence are laboratory ones: sperm sorting has a modest effect, and preimplantation genetic testing during IVF is essentially certain but is restricted to medical reasons in many countries, including the UK.
Is there any harm in trying the timing?
Aiming for the boy days is harmless because they are also the best days to conceive at all. Aiming for the girl days means avoiding ovulation day and the day before, which cuts your chance of conceiving that cycle by roughly half while doing nothing to the odds of a girl. If you are trying for a baby, cover the whole window.
What actually decides a baby's sex?
Which sperm fertilises the egg: an X-carrying sperm produces a girl, a Y-carrying sperm a boy. Roughly equal numbers of each are produced, and no reliable way has been found to tip the balance through timing or lifestyle. Worldwide about 51% of births are boys, a ratio that holds regardless of intercourse timing.
When can I find out the sex for real?
Non-invasive prenatal testing (NIPT) from about 10 weeks reads fetal DNA in the mother's blood and is over 99% accurate for sex. The anatomy ultrasound at 18 to 22 weeks is the usual point for most people. Some early scans at 12 to 14 weeks give a good guess.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.