Among healthy women trying to conceive, nearly all pregnancies can be attributed to intercourse during a six-day period ending on the day of ovulation. For practical purposes, the timing of sexual intercourse in relation to ovulation has no influence on the sex of the baby.

A 1995 study published in the New England Journal of Medicine concluded that "for practical purposes, the timing of sexual intercourse in relation to ovulation has no influence on the sex of the baby." The study found that cycles producing male and female babies had similar patterns of intercourse in relation to ovulation.

Evidence was insufficient to detect a benefit of timed intercourse using FABM on clinical pregnancy (RR 1.13, 95% CI 0.31 to 4.07, 1 RCT, n = 17, very low-quality evidence) or clinical or positive pregnancy test rates (RR 1.08, 95% CI 0.89 to 1.30, 3 RCTs, n = 262, very low-quality evidence). Due to limited data, we are uncertain of the effect of FABM in timed intercourse on pregnancy outcomes, adverse effects, and quality of life compared to intercourse without ovulation prediction.

A meta-analysis of six studies found a statistically significant lower proportion of male births among conceptions that occur during the most fertile time of the cycle, which disproves the suggestion that intercourse around ovulation selects for male offspring. The authors concluded that while there was a small but statistically significant deficit in male births during the most fertile period, it was insufficient to allow sex selection by timing of intercourse.

As of January 28, 2026, no scientific evidence supports that the frequency or timing of intercourse influences the sex of a baby. Gender is determined solely at conception by which sperm (X or Y chromosome) fertilizes the egg, a random biological event that cannot be manipulated by intercourse patterns.

Scientific research does not support timed intercourse methods, such as the Shettles and Whelan methods, as a reliable way to choose your baby's sex. A 1995 study found no reliable link between intercourse timing and a baby's sex, and following these methods may actually reduce overall chances of conceiving.

Unfortunately, the Shettles method isn't a scientifically-backed way to aim for a boy or a girl, says Noa Sterling, M.D., an ob-gyn in San Diego. This fundamental biological process is why most fertility specialists acknowledge that natural sex selection methods have limited scientific backing. A study 10 years later found the same results, then one published in 2019 went as far as to refute Shettles' assertion that X- and Y sperm behave differently based on their shape.

There is limited evidence to suggest the Shettles method works, with a 2016 study noting no evidence that the pattern of intercourse affects fetal sex, and a 2020 study indicating very little or no differences between sperm types beyond their DNA.

A clinical experimental trial conducted in Iran from 2013 to 2015, involving 285 participants, suggested that adjusting the time of sexual intercourse according to ovulation and observing a pre-pregnancy diet could be effective in sex selection, with reported success rates of 82.95% for the first group and 86.79% for the second group.

While the Shettles theory proposed that timing sex around ovulation could favor male sperm due to their speed, and earlier in the fertile window could favor female sperm due to their longevity, subsequent studies have shown that the timing of sex in relation to ovulation does not actually influence whether a couple conceives a boy or a girl.

The Shettles method of sex selection is based on the different environments that are prominent throughout the female reproductive tract during the menstrual cycle. The sixth edition of Your Baby's Sex: Now You Can Choose provided data from updated studies that showed a seventy-five percent success rate for couples that tried for females and an eighty percent success rate for couples that tried for males. However, after the sixth edition was published, another study was conducted on the shape of sperm cells that demonstrated that there were no shape differences between sperm carrying X or Y chromosomes.

Elizabeth Whelan, a public health researcher, suggested that couples only have intercourse at specific times during the woman's menstrual cycle based on whether they wanted a female or male infant. Since the publication of Whelan's book, many researchers have refuted her theories regarding the relationship between intercourse timing and an infant's sex. A study published in The New England Journal of Medicine in 1995 refuted all claims that intercourse timing affects the outcome of an infant's sex.

Evidence indicates that timing of coitus relative to ovulation influences a baby's sex. But, contrary to Shettles' method, coitus near ovulation is more likely to produce girls. Conception of a male child proved to be more likely (68%) when coitus and ovulation were separated by an interval greater than two days. By contrast, most female infants (64%) were conceived with a sperm present in the mother's reproductive tract for no more than a day.

The Shettles Method, developed by Landrum B. Shettles in the 1960s, proposes that couples can influence a baby's sex by timing intercourse relative to ovulation, based on the hypothesis that male (Y) sperm are faster but more fragile, while female (X) sperm are hardier. However, a 1995 article in the New England Journal of Medicine concluded that "for practical purposes, the timing of sexual intercourse in relation to ovulation has no influence on the sex of the baby."

By timing sex close to ovulation, you favor the Y chromosome sperm (boys) as they can swim quickly and reach the egg first. By timing sex further from ovulation (at the start of peak fertility only), you favor the X chromosome sperm (girls) who are larger, stronger, but swim more slowly. However, it should be made clear that trying to time intercourse to select gender means that you are reducing the days and frequency of lovemaking during your fertile window.